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Research Paper Immanuel Kant

Immanuel Kant: Metaphysics

Immanuel Kant (1724-1804) is one of the most influential philosophers in the history of Western philosophy. His contributions to metaphysics, epistemology, ethics, and aesthetics have had a profound impact on almost every philosophical movement that followed him. This article focuses on his metaphysics and epistemology in one of his most important works, The Critique of Pure Reason.  A large part of Kant’s work addresses the question “What can we know?” The answer, if it can be stated simply, is that our knowledge is constrained to mathematics and the science of the natural, empirical world. It is impossible, Kant argues, to extend knowledge to the supersensible realm of speculative metaphysics. The reason that knowledge has these constraints, Kant argues, is that the mind plays an active role in constituting the features of experience and limiting the mind’s access only to the empirical realm of space and time.

Kant responded to his predecessors by arguing against the Empiricists that the mind is not a blank slate that is written upon by the empirical world, and by rejecting the Rationalists’ notion that pure, a priori knowledge of a mind-independent world was possible.  Reason itself is structured with forms of experience and categories that give a phenomenal and logical structure to any possible object of empirical experience.  These categories cannot be circumvented to get at a mind-independent world, but they are necessary for experience of spatio-temporal objects with their causal behavior and logical properties.  These two theses constitute Kant’s famous transcendental idealism and empirical realism.

Kant’s contributions to ethics have been just as substantial, if not more so, than his work in metaphysics and epistemology.  He is the most important proponent in philosophical history of deontological, or duty based,  ethics. In Kant’s view, the sole feature that gives an action moral worth is not the outcome that is achieved by the action, but the motive that is behind the action.  And the only motive that can endow an act with moral value, he argues, is one that arises from universal principles discovered by reason.  The categorical imperative is Kant’s famous statement of this duty: “Act only according to that maxim by which you can at the same time will that it should become a universal law.”

Table of Contents

  1. Historical Background to Kant
    1. Empiricism
    2. Rationalism
  2. Kant's Answers to his Predecessors
  3. Kant's Copernican Revolution: Mind Making Nature
  4. Kant's Transcendental Idealism
  5. Kant's Analytic of Principles
  6. Kant's Dialectic
  7. The Ideas of Reason
  8. Kant's Ethics
    1. Reason and Freedom
    2. The Duality of the Human Situation
    3. The Good Will
    4. Duty
  9. Kant's Criticisms of Utilitarianism
  10. References and Further Reading

1. Historical Background to Kant

In order to understand Kant's position, we must understand the philosophical background that he was reacting to. First, this article presents a brief overview of his predecessor's positions with a brief statement of Kant's objections, then I will return to a more detailed exposition of Kant's arguments. There are two major historical movements in the early modern period of philosophy that had a significant impact on Kant: Empiricism and Rationalism. Kant argues that both the method and the content of these philosophers' arguments contain serious flaws. A central epistemological problem for philosophers in both movements was determining how we can escape from within the confines of the human mind and the immediately knowable content of our own thoughts to acquire knowledge of the world outside of us. The Empiricists sought to accomplish this through the senses and a posteriori reasoning. The Rationalists attempted to use a priori reasoning to build the necessary bridge. A posteriori reasoning depends upon experience or contingent events in the world to provide us with information. That "Bill Clinton was president of the United States in 1999," for example, is something that I can know only through experience; I cannot determine this to be true through an analysis of the concepts of "president" or "Bill Clinton." A priori reasoning, in contrast, does not depend upon experience to inform it. The concept "bachelor" logically entails the ideas of an unmarried, adult, human male without my needing to conduct a survey of bachelors and men who are unmarried. Kant believed that this twofold distinction in kinds of knowledge was inadequate to the task of understanding metaphysics for reasons we will discuss in a moment.

a. Empiricism

Empiricists, such as Locke, Berkeley, and Hume, argued that human knowledge originates in our sensations. Locke, for instance, was a representative realist about the external world and placed great confidence in the ability of the senses to inform us of the properties that empirical objects really have in themselves. Locke had also argued that the mind is a blank slate, or a tabula rasa, that becomes populated with ideas by its interactions with the world. Experience teaches us everything, including concepts of relationship, identity, causation, and so on. Kant argues that the blank slate model of the mind is insufficient to explain the beliefs about objects that we have; some components of our beliefs must be brought by the mind to experience.

Berkeley's strict phenomenalism, in contrast to Locke, raised questions about the inference from the character of our sensations to conclusions about the real properties of mind-independent objects. Since the human mind is strictly limited to the senses for its input, Berkeley argued, it has no independent means by which to verify the accuracy of the match between sensations and the properties that objects possess in themselves. In fact, Berkeley rejected the very idea of mind-independent objects on the grounds that a mind is, by its nature, incapable of possessing an idea of such a thing. Hence, in Kant's terms, Berkeley was a material idealist. To the material idealist, knowledge of material objects is ideal or unachievable, not real. For Berkeley, mind-independent material objects are impossible and unknowable. In our sense experience we only have access to our mental representations, not to objects themselves. Berkeley argues that our judgments about objects are really judgments about these mental representations alone, not the substance that gives rise to them. In the Refutation of Material Idealism, Kant argues that material idealism is actually incompatible with a position that Berkeley held, namely that we are capable of making judgments about our experience.

David Hume pursued Berkeley's empirical line of inquiry even further, calling into question even more of our common sense beliefs about the source and support of our sense perceptions. Hume maintains that we cannot provide a priori or a posteriori justifications for a number of our beliefs like, "Objects and subjects persist identically over time," or "Every event must have a cause." In Hume's hands, it becomes clear that empiricism cannot give us an epistemological justification for the claims about objects, subjects, and causes that we took to be most obvious and certain about the world.

Kant expresses deep dissatisfaction with the idealistic and seemingly skeptical results of the empirical lines of inquiry. In each case, Kant gives a number of arguments to show that Locke's, Berkeley's, and Hume's empiricist positions are untenable because they necessarily presuppose the very claims they set out to disprove. In fact, any coherent account of how we perform even the most rudimentary mental acts of self-awareness and making judgments about objects must presuppose these claims, Kant argues. Hence, while Kant is sympathetic with many parts of empiricism, ultimately it cannot be a satisfactory account of our experience of the world.

b. Rationalism

The Rationalists, principally Descartes, Spinoza, and Leibniz, approached the problems of human knowledge from another angle. They hoped to escape the epistemological confines of the mind by constructing knowledge of the external world, the self, the soul, God, ethics, and science out of the simplest, indubitable ideas possessed innately by the mind. Leibniz in particular, thought that the world was knowable a priori, through an analysis of ideas and derivations done through logic. Supersensible knowledge, the Rationalists argued, can be achieved by means of reason. Descartes believed that certain truths, that "if I am thinking, I exist," for example, are invulnerable to the most pernicious skepticism. Armed with the knowledge of his own existence, Descartes hoped to build a foundation for all knowledge.

Kant's Refutation of Material Idealism works against Descartes' project as well as Berkeley's. Descartes believed that he could infer the existence of objects in space outside of him based on his awareness of his own existence coupled with an argument that God exists and is not deceiving him about the evidence of his senses. Kant argues in the Refutation chapter that knowledge of external objects cannot be inferential. Rather, the capacity to be aware of one's own existence in Descartes' famous cogito argument already presupposes that existence of objects in space and time outside of me.

Kant had also come to doubt the claims of the Rationalists because of what he called Antinomies, or contradictory, but validly proven pairs of claims that reason is compelled toward. From the basic principles that the Rationalists held, it is possible, Kant argues, to prove conflicting claims like, "The world has a beginning in time and is limited as regards space," and "The world has no beginning, and no limits in space." (A 426/B 454) Kant claims that antinomies like this one reveal fundamental methodological and metaphysical mistakes in the rationalist project. The contradictory claims could both be proven because they both shared the mistaken metaphysical assumption that we can have knowledge of things as they are in themselves, independent of the conditions of our experience of them.

The Antinomies can be resolved, Kant argues, if we understand the proper function and domain of the various faculties that contribute to produce knowledge. We must recognize that we cannot know things as they are in themselves and that our knowledge is subject to the conditions of our experience. The Rationalist project was doomed to failure because it did not take note of the contribution that our faculty of reason makes to our experience of objects. Their a priori analysis of our ideas could inform us about the content of our ideas, but it could not give a coherent demonstration of metaphysical truths about the external world, the self, the soul, God, and so on.

2. Kant's Answers to his Predecessors

Kant's answer to the problems generated by the two traditions mentioned above changed the face of philosophy. First, Kant argued that that old division between a priori truths and a posteriori truths employed by both camps was insufficient to describe the sort of metaphysical claims that were under dispute. An analysis of knowledge also requires a distinction between synthetic and analytic truths. In an analytic claim, the predicate is contained within the subject. In the claim, "Every body occupies space," the property of occupying space is revealed in an analysis of what it means to be a body. The subject of a synthetic claim, however, does not contain the predicate. In, "This tree is 120 feet tall," the concepts are synthesized or brought together to form a new claim that is not contained in any of the individual concepts. The Empiricists had not been able to prove synthetic a priori claims like "Every event must have a cause," because they had conflated "synthetic" and "a posteriori" as well as "analytic" and "a priori." Then they had assumed that the two resulting categories were exhaustive. A synthetic a priori claim, Kant argues, is one that must be true without appealing to experience, yet the predicate is not logically contained within the subject, so it is no surprise that the Empiricists failed to produce the sought after justification. The Rationalists had similarly conflated the four terms and mistakenly proceeded as if claims like, "The self is a simple substance," could be proven analytically and a priori.

Synthetic a priori claims, Kant argues, demand an entirely different kind of proof than those required for analytic a priori claims or synthetic a posteriori claims. Indications for how to proceed, Kant says, can be found in the examples of synthetic a priori claims in natural science and mathematics, specifically geometry. Claims like Newton's, "the quantity of matter is always preserved," and the geometer's claim, "the angles of a triangle always add up to 180 degrees" are known a priori, but they cannot be known merely from an analysis of the concepts of matter or triangle. We must "go outside and beyond the concept. . . joining to it a priori in thought something which I have not thought in it." (B 18) A synthetic a priori claim constructs upon and adds to what is contained analytically in a concept without appealing to experience. So if we are to solve the problems generated by Empiricism and Rationalism, the central question of metaphysics in the Critique of Pure Reason reduces to "How are synthetic a priori judgments possible?" (19) (All references to The Critique of Pure Reason will be to the A (1781) and B(1787) edition pages in Werner Pluhar's translation. Indianapolis: Hackett, 1996.) If we can answer that question, then we can determine the possibility, legitimacy, and range of all metaphysical claims.

3. Kant's Copernican Revolution: Mind Making Nature

Kant's answer to the question is complicated, but his conclusion is that a number of synthetic a priori claims, like those from geometry and the natural sciences, are true because of the structure of the mind that knows them. "Every event must have a cause" cannot be proven by experience, but experience is impossible without it because it describes the way the mind must necessarily order its representations. We can understand Kant's argument again by considering his predecessors. According to the Rationalist and Empiricist traditions, the mind is passive either because it finds itself possessing innate, well-formed ideas ready for analysis, or because it receives ideas of objects into a kind of empty theater, or blank slate. Kant's crucial insight here is to argue that experience of a world as we have it is only possible if the mind provides a systematic structuring of its representations. This structuring is below the level of, or logically prior to, the mental representations that the Empiricists and Rationalists analyzed. Their epistemological and metaphysical theories could not adequately explain the sort of judgments or experience we have because they only considered the results of the mind's interaction with the world, not the nature of the mind's contribution. Kant's methodological innovation was to employ what he calls a transcendental argument to prove synthetic a priori claims. Typically, a transcendental argument attempts to prove a conclusion about the necessary structure of knowledge on the basis of an incontrovertible mental act. Kant argues in the Refutation of Material Idealism that the fact that "There are objects that exist in space and time outside of me," (B 274) which cannot be proven by a priori or a posteriori methods, is a necessary condition of the possibility of being aware of one's own existence. It would not be possible to be aware of myself as existing, he says, without presupposing the existing of something permanent outside of me to distinguish myself from. I am aware of myself as existing. Therefore, there is something permanent outside of me.

This argument is one of many transcendental arguments that Kant gives that focuses on the contribution that the mind itself makes to its experience. These arguments lead Kant to reject the Empiricists' assertion that experience is the source of all our ideas. It must be the mind's structuring, Kant argues, that makes experience possible. If there are features of experience that the mind brings to objects rather than given to the mind by objects, that would explain why they are indispensable to experience but unsubstantiated in it. And that would explain why we can give a transcendental argument for the necessity of these features. Kant thought that Berkeley and Hume identified at least part of the mind's a priori contribution to experience with the list of claims that they said were unsubstantiated on empirical grounds: "Every event must have a cause," "There are mind-independent objects that persist over time," and "Identical subjects persist over time." The empiricist project must be incomplete since these claims are necessarily presupposed in our judgments, a point Berkeley and Hume failed to see. So, Kant argues that a philosophical investigation into the nature of the external world must be as much an inquiry into the features and activity of the mind that knows it.

The idea that the mind plays an active role in structuring reality is so familiar to us now that it is difficult for us to see what a pivotal insight this was for Kant. He was well aware of the idea's power to overturn the philosophical worldviews of his contemporaries and predecessors, however. He even somewhat immodestly likens his situation to that of Copernicus in revolutionizing our worldview. In the Lockean view, mental content is given to the mind by the objects in the world. Their properties migrate into the mind, revealing the true nature of objects. Kant says, "Thus far it has been assumed that all our cognition must conform to objects" (B xvi). But that approach cannot explain why some claims like, "every event must have a cause," are a priori true. Similarly, Copernicus recognized that the movement of the stars cannot be explained by making them revolve around the observer; it is the observer that must be revolving. Analogously, Kant argued that we must reformulate the way we think about our relationship to objects. It is the mind itself which gives objects at least some of their characteristics because they must conform to its structure and conceptual capacities. Thus, the mind's active role in helping to create a world that is experiencable must put it at the center of our philosophical investigations. The appropriate starting place for any philosophical inquiry into knowledge, Kant decides, is with the mind that can have that knowledge.

Kant's critical turn toward the mind of the knower is ambitious and challenging. Kant has rejected the dogmatic metaphysics of the Rationalists that promises supersensible knowledge. And he has argued that Empiricism faces serious limitations. His transcendental method will allow him to analyze the metaphysical requirements of the empirical method without venturing into speculative and ungrounded metaphysics. In this context, determining the "transcendental" components of knowledge means determining, "all knowledge which is occupied not so much with objects as with the mode of our knowledge of objects in so far as this mode of knowledge is to be possible a priori." (A 12/B 25)

The project of the Critique of Pure Reason is also challenging because in the analysis of the mind's transcendental contributions to experience we must employ the mind, the only tool we have, to investigate the mind. We must use the faculties of knowledge to determine the limits of knowledge, so Kant's Critique of Pure Reason is both a critique that takes pure reason as its subject matter, and a critique that is conducted by pure reason.

Kant's argument that the mind makes an a priori contribution to experiences should not be mistaken for an argument like the Rationalists' that the mind possesses innate ideas like, "God is a perfect being." Kant rejects the claim that there are complete propositions like this one etched on the fabric of the mind. He argues that the mind provides a formal structuring that allows for the conjoining of concepts into judgments, but that structuring itself has no content. The mind is devoid of content until interaction with the world actuates these formal constraints. The mind possesses a priori templates for judgments, not a priori judgments.

4. Kant's Transcendental Idealism

With Kant's claim that the mind of the knower makes an active contribution to experience of objects before us, we are in a better position to understand transcendental idealism.  Kant's arguments are designed to show the limitations of our knowledge. The Rationalists believed that we could possess metaphysical knowledge about God, souls, substance, and so forth; they believed such knowledge was transcendentally real. Kant argues, however, that we cannot have knowledge of the realm beyond the empirical. That is, transcendental knowledge is ideal, not real, for minds like ours. Kant identifies two a priori sources of these constraints. The mind has a receptive capacity, or the sensibility, and the mind possesses a conceptual capacity, or the understanding.

In the Transcendental Aesthetic section of the Critique, Kant argues that sensibility is the understanding's means of accessing objects. The reason synthetic a priori judgments are possible in geometry, Kant argues, is that space is an a priori form of sensibility. That is, we can know the claims of geometry with a priori certainty (which we do) only if experiencing objects in space is the necessary mode of our experience. Kant also argues that we cannot experience objects without being able to represent them spatially. It is impossible to grasp an object as an object unless we delineate the region of space it occupies. Without a spatial representation, our sensations are undifferentiated and we cannot ascribe properties to particular objects. Time, Kant argues, is also necessary as a form or condition of our intuitions of objects. The idea of time itself cannot be gathered from experience because succession and simultaneity of objects, the phenomena that would indicate the passage of time, would be impossible to represent if we did not already possess the capacity to represent objects in time.

Another way to understand Kant's point here is that it is impossible for us to have any experience of objects that are not in time and space. Furthermore, space and time themselves cannot be perceived directly, so they must be the form by which experience of objects is had. A consciousness that apprehends objects directly, as they are in themselves and not by means of space and time, is possible—God, Kant says, has a purely intuitive consciousness—but our apprehension of objects is always mediated by the conditions of sensibility. Any discursive or concept using consciousness (A 230/B 283) like ours must apprehend objects as occupying a region of space and persisting for some duration of time.

Subjecting sensations to the a priori conditions of space and time is not sufficient to make judging objects possible. Kant argues that the understanding must provide the concepts, which are rules for identifying what is common or universal in different representations.(A 106) He says, "without sensibility no object would be given to us; and without understanding no object would be thought. Thoughts without content are empty; intuitions without concepts are blind." (B 75) Locke's mistake was believing that our sensible apprehensions of objects are thinkable and reveal the properties of the objects themselves. In the Analytic of Concepts section of the Critique, Kant argues that in order to think about the input from sensibility, sensations must conform to the conceptual structure that the mind has available to it. By applying concepts, the understanding takes the particulars that are given in sensation and identifies what is common and general about them. A concept of "shelter" for instance, allows me to identify what is common in particular representations of a house, a tent, and a cave.

The empiricist might object at this point by insisting that such concepts do arise from experience, raising questions about Kant's claim that the mind brings an a priori conceptual structure to the world. Indeed, concepts like "shelter" do arise partly from experience. But Kant raises a more fundamental issue. An empirical derivation is not sufficient to explain all of our concepts. As we have seen, Hume argued, and Kant accepts, that we cannot empirically derive our concepts of causation, substance, self, identity, and so forth. What Hume had failed to see, Kant argues, is that even the possibility of making judgments about objects, to which Hume would assent, presupposes the possession of these fundamental concepts. Hume had argued for a sort of associationism to explain how we arrive at causal beliefs. My idea of a moving cue ball, becomes associated with my idea of the eight ball that is struck and falls into the pocket. Under the right circumstances, repeated impressions of the second following the first produces a belief in me that the first causes the second.

The problem that Kant points out is that a Humean association of ideas already presupposes that we can conceive of identical, persistent objects that have regular, predictable, causal behavior. And being able to conceive of objects in this rich sense presupposes that the mind makes several a priori contributions. I must be able to separate the objects from each other in my sensations, and from my sensations of myself. I must be able to attribute properties to the objects. I must be able to conceive of an external world with its own course of events that is separate from the stream of perceptions in my consciousness. These components of experience cannot be found in experience because they constitute it. The mind's a priori conceptual contribution to experience can be enumerated by a special set of concepts that make all other empirical concepts and judgments possible. These concepts cannot be experienced directly; they are only manifest as the form which particular judgments of objects take. Kant believes that formal logic has already revealed what the fundamental categories of thought are. The special set of concepts is Kant's Table of Categories, which are taken mostly from Aristotle with a few revisions:

Of Quantity
Unity
Plurality
Totality
Of QualityOf Relation
RealityInherence and Subsistence
NegationCausality and Dependence
LimitationCommunity
Of Modality
Possibility-Impossibility
Existence-Nonexistence
Necessity-Contingency

While Kant does not give a formal derivation of it, he believes that this is the complete and necessary list of the a priori contributions that the understanding brings to its judgments of the world. Every judgment that the understanding can make must fall under the table of categories. And subsuming spatiotemporal sensations under the formal structure of the categories makes judgments, and ultimately knowledge, of empirical objects possible.

Since objects can only be experienced spatiotemporally, the only application of concepts that yields knowledge is to the empirical, spatiotemporal world. Beyond that realm, there can be no sensations of objects for the understanding to judge, rightly or wrongly. Since intuitions of the physical world are lacking when we speculate about what lies beyond, metaphysical knowledge, or knowledge of the world outside the physical, is impossible. Claiming to have knowledge from the application of concepts beyond the bounds of sensation results in the empty and illusory transcendent metaphysics of Rationalism that Kant reacts against.

It should be pointed out, however, that Kant is not endorsing an idealism about objects like Berkeley's. That is, Kant does not believe that material objects are unknowable or impossible. While Kant is a transcendental idealist--he believes the nature of objects as they are in themselves is unknowable to us--knowledge of appearances is nevertheless possible. As noted above, in The Refutation of Material Idealism, Kant argues that the ordinary self-consciousness that Berkeley and Descartes would grant implies "the existence of objects in space outside me." (B 275) Consciousness of myself would not be possible if I were not able to make determinant judgments about objects that exist outside of me and have states that are independent of my inner experience. Another way to put the point is to say that the fact that the mind of the knower makes the a priori contribution does not mean that space and time or the categories are mere figments of the imagination. Kant is an empirical realist about the world we experience; we can know objects as they appear to us. He gives a robust defense of science and the study of the natural world from his argument about the mind's role in making nature. All discursive, rational beings must conceive of the physical world as spatially and temporally unified, he argues. And the table of categories is derived from the most basic, universal forms of logical inference, Kant believes. Therefore, it must be shared by all rational beings. So those beings also share judgments of an intersubjective, unified, public realm of empirical objects. Hence, objective knowledge of the scientific or natural world is possible. Indeed, Kant believes that the examples of Newton and Galileo show it is actual. So Berkeley's claims that we do not know objects outside of us and that such knowledge is impossible are both mistaken.

In conjunction with his analysis of the possibility of knowing empirical objects, Kant gives an analysis of the knowing subject that has sometimes been called his transcendental psychology. Much of Kant's argument can be seen as subjective, not because of variations from mind to mind, but because the source of necessity and universality is in the mind of the knowing subject, not in objects themselves. Kant draws several conclusions about what is necessarily true of any consciousness that employs the faculties of sensibility and understanding to produce empirical judgments. As we have seen, a mind that employs concepts must have a receptive faculty that provides the content of judgments. Space and time are the necessary forms of apprehension for the receptive faculty. The mind that has experience must also have a faculty of combination or synthesis, the imagination for Kant, that apprehends the data of sense, reproduces it for the understanding, and recognizes their features according to the conceptual framework provided by the categories. The mind must also have a faculty of understanding that provides empirical concepts and the categories for judgment. The various faculties that make judgment possible must be unified into one mind. And it must be identical over time if it is going to apply its concepts to objects over time. Kant here addresses Hume's famous assertion that introspection reveals nothing more than a bundle of sensations that we group together and call the self. Judgments would not be possible, Kant maintains, if the mind that senses is not the same as the mind that possesses the forms of sensibility. And that mind must be the same as the mind that employs the table of categories, that contributes empirical concepts to judgment, and that synthesizes the whole into knowledge of a unified, empirical world. So the fact that we can empirically judge proves, contra Hume, that the mind cannot be a mere bundle of disparate introspected sensations. In his works on ethics Kant will also argue that this mind is the source of spontaneous, free, and moral action. Kant believes that all the threads of his transcendental philosophy come together in this "highest point" which he calls the transcendental unity of apperception.

5. Kant's Analytic of Principles

We have seen the progressive stages of Kant's analysis of the faculties of the mind which reveals the transcendental structuring of experience performed by these faculties. First, in his analysis of sensibility, he argues for the necessarily spatiotemporal character of sensation. Then Kant analyzes the understanding, the faculty that applies concepts to sensory experience. He concludes that the categories provide a necessary, foundational template for our concepts to map onto our experience. In addition to providing these transcendental concepts, the understanding also is the source of ordinary empirical concepts that make judgments about objects possible. The understanding provides concepts as the rules for identifying the properties in our representations.

Kant's next concern is with the faculty of judgment, "If understanding as such is explicated as our power of rules, then the power of judgment is the ability to subsume under rules, i.e., to distinguish whether something does or does not fall under a given rule." (A 132/B 172). The next stage in Kant's project will be to analyze the formal or transcendental features of experience that enable judgment, if there are any such features besides what the previous stages have identified. The cognitive power of judgment does have a transcendental structure. Kant argues that there are a number of principles that must necessarily be true of experience in order for judgment to be possible. Kant's analysis of judgment and the arguments for these principles are contained in his Analytic of Principles.

Within the Analytic, Kant first addresses the challenge of subsuming particular sensations under general categories in the Schematism section. Transcendental schemata, Kant argues, allow us to identify the homogeneous features picked out by concepts from the heterogeneous content of our sensations. Judgment is only possible if the mind can recognize the components in the diverse and disorganized data of sense that make those sensations an instance of a concept or concepts. A schema makes it possible, for instance, to subsume the concrete and particular sensations of an Airedale, a Chihuahua, and a Labrador all under the more abstract concept "dog."

The full extent of Kant's Copernican revolution becomes even more clear in the rest of the Analytic of Principles. That is, the role of the mind in making nature is not limited to space, time, and the categories. In the Analytic of Principles, Kant argues that even the necessary conformity of objects to natural law arises from the mind. Thus far, Kant's transcendental method has permitted him to reveal the a priori components of sensations, the a priori concepts. In the sections titled the Axioms, Anticipations, Analogies, and Postulates, he argues that there are a priori judgments that must necessarily govern all appearances of objects. These judgments are a function of the table of categories' role in determining all possible judgments, so the four sections map onto the four headings of that table. I include all of the a priori judgments, or principles, here to illustrate the earlier claims about Kant's empirical realism, and to show the intimate relationship Kant saw between his project and that of the natural sciences:

Axioms of Intuition
All intuitions are extensive magnitudes.
Anticipations of PerceptionAnalogies of Experience
In all appearances the real that is an object of sensation has intensive magnitude, i.e., a degree.In all variations by appearances substance is permanent, and its quantum in nature is neither increased nor decreased.
All changes occur according to the law of the connection of cause and effect.
All substances, insofar as they can be perceived in space as simultaneous, are in thoroughgoing interaction.
Postulates of Empirical Thought
What agrees (in terms of intuition and concepts) with the formal conditions of experience is possible.
What coheres with the material conditions of experience (with sensation) is actual.
That whose coherence with the actual is determined according to universal conditions of experience is necessary (exists necessarily)

6. Kant's Dialectic

The discussion of Kant's metaphysics and epistemology so far (including the Analytic of Principles) has been confined primarily to the section of the Critique of Pure Reason that Kant calls the Transcendental Analytic. The purpose of the Analytic, we are told, is "the rarely attempted dissection of the power of the understanding itself." (A 65/B 90). Kant's project has been to develop the full argument for his theory about the mind's contribution to knowledge of the world. Once that theory is in place, we are in a position to see the errors that are caused by transgressions of the boundaries to knowledge established by Kant's transcendental idealism and empirical realism. Kant calls judgments that pretend to have knowledge beyond these boundaries and that even require us to tear down the limits that he has placed on knowledge, transcendent judgments. The Transcendental Dialectic section of the book is devoted to uncovering the illusion of knowledge created by transcendent judgments and explaining why the temptation to believe them persists. Kant argues that the proper functioning of the faculties of sensibility and the understanding combine to draw reason, or the cognitive power of inference, inexorably into mistakes. The faculty of reason naturally seeks the highest ground of unconditional unity. It seeks to unify and subsume all particular experiences under higher and higher principles of knowledge. But sensibility cannot by its nature provide the intuitions that would make knowledge of the highest principles and of things as they are in themselves possible. Nevertheless, reason, in its function as the faculty of inference, inevitably draws conclusions about what lies beyond the boundaries of sensibility. The unfolding of this conflict between the faculties reveals more about the mind's relationship to the world it seeks to know and the possibility of a science of metaphysics.

Kant believes that Aristotle's logic of the syllogism captures the logic employed by reason. The resulting mistakes from the inevitable conflict between sensibility and reason reflect the logic of Aristotle's syllogism. Corresponding to the three basic kinds of syllogism are three dialectic mistakes or illusions of transcendent knowledge that cannot be real. Kant's discussion of these three classes of mistakes are contained in the Paralogisms, the Antinomies, and the Ideals of Reason. The Dialectic explains the illusions of reason in these sections. But since the illusions arise from the structure of our faculties, they will not cease to have their influence on our minds any more than we can prevent the moon from seeming larger when it is on the horizon than when it is overhead. (A 297/B 354).

In the Paralogisms, Kant argues that a failure to recognize the difference between appearances and things in themselves, particularly in the case of the introspected self, leads us into transcendent error. Kant argues against several conclusions encouraged by Descartes and the rational psychologists, who believed they could build human knowledge from the "I think" of the cogito argument. From the "I think" of self-awareness we can infer, they maintain, that the self or soul is 1) simple, 2) immaterial, 3) an identical substance and 4) that we perceive it directly, in contrast to external objects whose existence is merely possible. That is, the rational psychologists claimed to have knowledge of the self as transcendentally real. Kant believes that it is impossible to demonstrate any of these four claims, and that the mistaken claims to knowledge stem from a failure to see the real nature of our apprehension of the "I." Reason cannot fail to apply the categories to its judgments of the self, and that application gives rise to these four conclusions about the self that correspond roughly to the four headings in the table of categories. But to take the self as an object of knowledge here is to pretend to have knowledge of the self as it is in itself, not as it appears to us. Our representation of the "I" itself is empty. It is subject to the condition of inner sense, time, but not the condition of outer sense, space, so it cannot be a proper object of knowledge. It can be thought through concepts, but without the commensurate spatial and temporal intuitions, it cannot be known. Each of the four paralogisms explains the categorical structure of reason that led the rational psychologists to mistake the self as it appears to us for the self as it is in itself.

We have already mentioned the Antinomies, in which Kant analyzes the methodological problems of the Rationalist project. Kant sees the Antinomies as the unresolved dialogue between skepticism and dogmatism about knowledge of the world. There are four antinomies, again corresponding to the four headings of the table of categories, that are generated by reason's attempts to achieve complete knowledge of the realm beyond the empirical. Each antinomy has a thesis and an antithesis, both of which can be validly proven, and since each makes a claim that is beyond the grasp of spatiotemporal sensation, neither can be confirmed or denied by experience. The First Antinomy argues both that the world has a beginning in time and space, and no beginning in time and space. The Second Antinomy's arguments are that every composite substance is made of simple parts and that nothing is composed of simple parts. The Third Antinomy's thesis is that agents like ourselves have freedom and its antithesis is that they do not. The Fourth Antinomy contains arguments both for and against the existence of a necessary being in the world. The seemingly irreconcilable claims of the Antinomies can only be resolved by seeing them as the product of the conflict of the faculties and by recognizing the proper sphere of our knowledge in each case. In each of them, the idea of "absolute totality, which holds only as a condition of things in themselves, has been applied to appearances" (A 506/B534).

The result of Kant' analysis of the Antinomies is that we can reject both claims of the first two and accept both claims of the last two, if we understand their proper domains. In the first Antinomy, the world as it appears to us is neither finite since we can always inquire about its beginning or end, nor is it infinite because finite beings like ourselves cannot cognize an infinite whole. As an empirical object, Kant argues, it is indefinitely constructable for our minds. As it is in itself, independent of the conditions of our thought, it should not be identified as finite or infinite since both are categorical conditions of our thought. Kant's resolution of the third Antinomy (A 445/B 473) clarifies his position on freedom. He considers the two competing hypotheses of speculative metaphysics that there are different types of causality in the world: 1) there are natural causes which are themselves governed by the laws of nature as well as uncaused causes like ourselves that can act freely, or 2) the causal laws of nature entirely govern the world including our actions. The conflict between these contrary claims can be resolved, Kant argues, by taking his critical turn and recognizing that it is impossible for any cause to be thought of as uncaused itself in the realm of space and time. But reason, in trying to understand the ground of all things, strives to unify its knowledge beyond the empirical realm. The empirical world, considered by itself, cannot provide us with ultimate reasons. So if we do not assume a first or free cause we cannot completely explain causal series in the world. So for the Third Antinomy, as for all of the Antinomies, the domain of the Thesis is the intellectual, rational, noumenal world. The domain of the Antithesis is the spatiotemporal world.

7. The Ideas of Reason

The faculty of reason has two employments. For the most part, we have engaged in an analysis of theoretical reason which has determined the limits and requirements of the employment of the faculty of reason to obtain knowledge. Theoretical reason, Kant says, makes it possible to cognize what is. But reason has its practical employment in determining what ought to be as well. (A 633/B 661) This distinction roughly corresponds to the two philosophical enterprises of metaphysics and ethics. Reason's practical use is manifest in the regulative function of certain concepts that we must think with regard to the world, even though we can have no knowledge of them.

Kant believes that, "Human reason is by its nature architectonic." (A 474/B 502). That is, reason thinks of all cognitions as belonging to a unified and organized system. Reason is our faculty of making inferences and of identifying the grounds behind every truth. It allows us to move from the particular and contingent to the global and universal. I infer that "Caius is mortal" from the fact that "Caius is a man" and the universal claim, "All men are mortal." In this fashion, reason seeks higher and higher levels of generality in order to explain the way things are. In a different kind of example, the biologist's classification of every living thing into a kingdom, phylum, class, order, family, genus, and species, illustrates reason's ambition to subsume the world into an ordered, unified system. The entire empirical world, Kant argues, must be conceived of by reason as causally necessitated (as we saw in the Analogies). We must connect, "one state with a previous state upon which the state follows according to a rule." Each cause, and each cause's cause, and each additional ascending cause must itself have a cause. Reason generates this hierarchy that combines to provide the mind with a conception of a whole system of nature. Kant believes that it is part of the function of reason to strive for a complete, determinate understanding of the natural world. But our analysis of theoretical reason has made it clear that we can never have knowledge of the totality of things because we cannot have the requisite sensations of the totality, hence one of the necessary conditions of knowledge is not met. Nevertheless, reason seeks a state of rest from the regression of conditioned, empirical judgments in some unconditioned ground that can complete the series (A 584/B 612). Reason's structure pushes us to accept certain ideas of reason that allow completion of its striving for unity. We must assume the ideas of God, freedom, and immortality, Kant says, not as objects of knowledge, but as practical necessities for the employment of reason in the realm where we can have knowledge. By denying the possibility of knowledge of these ideas, yet arguing for their role in the system of reason, Kant had to, "annul knowledge in order to make room for faith." (B xxx).

8. Kant's Ethics

It is rare for a philosopher in any era to make a significant impact on any single topic in philosophy. For a philosopher to impact as many different areas as Kant did is extraordinary. His ethical theory has been as influential as, if not more influential than, his work in epistemology and metaphysics. Most of Kant's work on ethics is presented in two works. The Foundations of the Metaphysics of Morals (1785) is Kant's "search for and establishment of the supreme principle of morality." In The Critique of Practical Reason (1787) Kant attempts to unify his account of practical reason with his work in the Critique of Pure Reason. Kant is the primary proponent in history of what is called deontological ethics. Deontology is the study of duty. On Kant's view, the sole feature that gives an action moral worth is not the outcome that is achieved by the action, but the motive that is behind the action. The categorical imperative is Kant's famous statement of this duty: "Act only according to that maxim by which you can at the same time will that it should become a universal law."

a. Reason and Freedom

For Kant, as we have seen, the drive for total, systematic knowledge in reason can only be fulfilled with assumptions that empirical observation cannot support. The metaphysical facts about the ultimate nature of things in themselves must remain a mystery to us because of the spatiotemporal constraints on sensibility. When we think about the nature of things in themselves or the ultimate ground of the empirical world, Kant has argued that we are still constrained to think through the categories, we cannot think otherwise, but we can have no knowledge because sensation provides our concepts with no content. So, reason is put at odds with itself because it is constrained by the limits of its transcendental structure, but it seeks to have complete knowledge that would take it beyond those limits.

Freedom plays a central role in Kant's ethics because the possibility of moral judgments presupposes it. Freedom is an idea of reason that serves an indispensable practical function. Without the assumption of freedom, reason cannot act. If we think of ourselves as completely causally determined, and not as uncaused causes ourselves, then any attempt to conceive of a rule that prescribes the means by which some end can be achieved is pointless. I cannot both think of myself as entirely subject to causal law and as being able to act according to the conception of a principle that gives guidance to my will. We cannot help but think of our actions as the result of an uncaused cause if we are to act at all and employ reason to accomplish ends and understand the world.

So reason has an unavoidable interest in thinking of itself as free. That is, theoretical reason cannot demonstrate freedom, but practical reason must assume it for the purpose of action. Having the ability to make judgments and apply reason puts us outside that system of causally necessitated events. "Reason creates for itself the idea of a spontaneity that can, on its own, start to act--without, i.e., needing to be preceded by another cause by means of which it is determined to action in turn, according to the law of causal connection," Kant says. (A 533/B 561) In its intellectual domain, reason must think of itself as free.

It is dissatisfying that he cannot demonstrate freedom; nevertheless, it comes as no surprise that we must think of ourselves as free. In a sense, Kant is agreeing with the common sense view that how I choose to act makes a difference in how I actually act. Even if it were possible to give a predictive empirical account of why I act as I do, say on the grounds of a functionalist psychological theory, those considerations would mean nothing to me in my deliberations. When I make a decision about what to do, about which car to buy, for instance, the mechanism at work in my nervous system makes no difference to me. I still have to peruse Consumer Reports, consider my options, reflect on my needs, and decide on the basis of the application of general principles. My first person perspective is unavoidable, hence the deliberative, intellectual process of choice is unavoidable.

b. The Duality of the Human Situation

The question of moral action is not an issue for two classes of beings, according to Kant. The animal consciousness, the purely sensuous being, is entirely subject to causal determination. It is part of the causal chains of the empirical world, but not an originator of causes the way humans are. Hence, rightness or wrongness, as concepts that apply to situations one has control over, do not apply. We do not morally fault the lion for killing the gazelle, or even for killing its own young. The actions of a purely rational being, by contrast, are in perfect accord with moral principles, Kant says. There is nothing in such a being's nature to make it falter. Its will always conforms with the dictates of reason. Humans are between the two worlds. We are both sensible and intellectual, as was pointed out in the discussion of the first Critique. We are neither wholly determined to act by natural impulse, nor are we free of non-rational impulse. Hence we need rules of conduct. We need, and reason is compelled to provide, a principle that declares how we ought to act when it is in our power to choose

Since we find ourselves in the situation of possessing reason, being able to act according to our own conception of rules, there is a special burden on us. Other creatures are acted upon by the world. But having the ability to choose the principle to guide our actions makes us actors. We must exercise our will and our reason to act. Will is the capacity to act according to the principles provided by reason. Reason assumes freedom and conceives of principles of action in order to function.

Two problems face us however. First, we are not wholly rational beings, so we are liable to succumb to our non-rational impulses. Second, even when we exercise our reason fully, we often cannot know which action is the best. The fact that we can choose between alternate courses of actions (we are not determined to act by instinct or reason) introduces the possibility that there can be better or worse ways of achieving our ends and better or worse ends, depending upon the criteria we adopt. The presence of two different kinds of object in the world adds another dimension, a moral dimension, to our deliberations. Roughly speaking, we can divide the world into beings with reason and will like ourselves and things that lack those faculties. We can think of these classes of things as ends-in-themselves and mere means-to-ends, respectively. Ends-in-themselves are autonomous beings with their own agendas; failing to recognize their capacity to determine their own actions would be to thwart their freedom and undermine reason itself. When we reflect on alternative courses of action, means-to-ends, things like buildings, rocks, and trees, deserve no special status in our deliberations about what goals we should have and what means we use to achieve them. The class of ends-in-themselves, reasoning agents like ourselves, however, do have a special status in our considerations about what goals we should have and the means we employ to accomplish them. Moral actions, for Kant, are actions where reason leads, rather than follows, and actions where we must take other beings that act according to their own conception of the law into account.

c. The Good Will

The will, Kant says, is the faculty of acting according to a conception of law. When we act, whether or not we achieve what we intend with our actions is often beyond our control, so the morality of our actions does not depend upon their outcome. What we can control, however, is the will behind the action. That is, we can will to act according to one law rather than another. The morality of an action, therefore, must be assessed in terms of the motivation behind it. If two people, Smith and Jones, perform the same act, from the same conception of the law, but events beyond Smith's control prevent her from achieving her goal, Smith is not less praiseworthy for not succeeding. We must consider them on equal moral ground in terms of the will behind their actions.

The only thing that is good without qualification is the good will, Kant says. All other candidates for an intrinsic good have problems, Kant argues. Courage, health, and wealth can all be used for ill purposes, Kant argues, and therefore cannot be intrinsically good. Happiness is not intrinsically good because even being worthy of happiness, Kant says, requires that one possess a good will. The good will is the only unconditional good despite all encroachments. Misfortune may render someone incapable of achieving her goals, for instance, but the goodness of her will remains.

Goodness cannot arise from acting on impulse or natural inclination, even if impulse coincides with duty. It can only arise from conceiving of one's actions in a certain way. A shopkeeper, Kant says, might do what is in accord with duty and not overcharge a child. Kant argues, "it is not sufficient to do that which should be morally good that it conform to the law; it must be done for the sake of the law." (Foundations of the Metaphysics of Morals,Akademie pagination 390) There is a clear moral difference between the shopkeeper that does it for his own advantage to keep from offending other customers and the shopkeeper who does it from duty and the principle of honesty.(Ibid., 398) Likewise, in another of Kant's carefully studied examples, the kind act of the person who overcomes a natural lack of sympathy for other people out of respect for duty has moral worth, whereas the same kind act of the person who naturally takes pleasure in spreading joy does not. A person's moral worth cannot be dependent upon what nature endowed them with accidentally. The selfishly motivated shopkeeper and the naturally kind person both act on equally subjective and accidental grounds. What matters to morality is that the actor think about their actions in the right manner.

We might be tempted to think that the motivation that makes an action good is having a positive goal--to make people happy, or to provide some benefit. But that is not the right sort of motive, Kant says. No outcome, should we achieve it, can be unconditionally good. Fortune can be misused, what we thought would induce benefit might actually bring harm, and happiness might be undeserved. Hoping to achieve some particular end, no matter how beneficial it may seem, is not purely and unconditionally good. It is not the effect or even the intended effect that bestows moral character on an action. All intended effects "could be brought about through other causes and would not require the will of a rational being, while the highest and unconditional good can be found only in such a will." (Ibid., 401) It is the possession of a rationally guided will that adds a moral dimension to one's acts. So it is the recognition and appreciation of duty itself that must drive our actions.

d. Duty

What is the duty that is to motivate our actions and to give them moral value? Kant distinguishes two kinds of law produced by reason. Given some end we wish to achieve, reason can provide a hypothetical imperative, or rule of action for achieving that end. A hypothetical imperative says that if you wish to buy a new car, then you must determine what sort of cars are available for purchase. Conceiving of a means to achieve some desired end is by far the most common employment of reason. But Kant has shown that the acceptable conception of the moral law cannot be merely hypothetical. Our actions cannot be moral on the ground of some conditional purpose or goal. Morality requires an unconditional statement of one's duty.

And in fact, reason produces an absolute statement of moral action. The moral imperative is unconditional; that is, its imperative force is not tempered by the conditional "if I want to achieve some end, then do X." It simply states, do X. Kant believes that reason dictates a categorical imperative for moral action. He gives at least three formulations of the Categorical Imperative.

  1. "Act only according to that maxim by which you can at the same time will that it should become a universal law." (Ibid., 422)
  2. "Act as though the maxim of your action were by your will to become a universal law of nature." (Ibid)
  3. Act so that you treat humanity, whether in your own person or in that of another, always as an end and never as a means only." (Ibid., 429)

What are Kant's arguments for the Categorical Imperative? First, consider an example. Consider the person who needs to borrow money and is considering making a false promise to pay it back. The maxim that could be invoked is, "when I need of money, borrow it, promising to repay it, even though I do not intend to." But when we apply the universality test to this maxim it becomes clear that if everyone were to act in this fashion, the institution of promising itself would be undermined. The borrower makes a promise, willing that there be no such thing as promises. Thus such an action fails the universality test.

The argument for the first formulation of the categorical imperative can be thought of this way. We have seen that in order to be good, we must remove inclination and the consideration of any particular goal from our motivation to act. The act cannot be good if it arises from subjective impulse. Nor can it be good because it seeks after some particular goal which might not attain the good we seek or could come about through happenstance. We must abstract away from all hoped for effects. If we remove all subjectivity and particularity from motivation we are only left with will to universality. The question "what rule determines what I ought to do in this situation?" becomes "what rule ought to universally guide action?" What we must do in any situation of moral choice is act according to a maxim that we would will everyone to act according to.

The second version of the Categorical Imperative invokes Kant's conception of nature and draws on the first Critique. In the earlier discussion of nature, we saw that the mind necessarily structures nature. And reason, in its seeking of ever higher grounds of explanation, strives to achieve unified knowledge of nature. A guide for us in moral matters is to think of what would not be possible to will universally. Maxims that fail the test of the categorical imperative generate a contradiction. Laws of nature cannot be contradictory. So if a maxim cannot be willed to be a law of nature, it is not moral.

The third version of the categorical imperative ties Kant's whole moral theory together. Insofar as they possess a rational will, people are set off in the natural order of things. They are not merely subject to the forces that act upon them; they are not merely means to ends. They are ends in themselves. All means to an end have a merely conditional worth because they are valuable only for achieving something else. The possessor of a rational will, however, is the only thing with unconditional worth. The possession of rationality puts all beings on the same footing, "every other rational being thinks of his existence by means of the same rational ground which holds also for myself; thus it is at the same time an objective principle from which, as a supreme practical ground, it must be possible to derive all laws of the will." (Ibid., 429)

9. Kant's Criticisms of Utilitarianism

Kant's criticisms of utilitarianism have become famous enough to warrant some separate discussion. Utilitarian moral theories evaluate the moral worth of action on the basis of happiness that is produced by an action. Whatever produces the most happiness in the most people is the moral course of action. Kant has an insightful objection to moral evaluations of this sort. The essence of the objection is that utilitarian theories actually devalue the individuals it is supposed to benefit. If we allow utilitarian calculations to motivate our actions, we are allowing the valuation of one person's welfare and interests in terms of what good they can be used for. It would be possible, for instance, to justify sacrificing one individual for the benefits of others if the utilitarian calculations promise more benefit. Doing so would be the worst example of treating someone utterly as a means and not as an end in themselves.

Another way to consider his objection is to note that utilitarian theories are driven by the merely contingent inclination in humans for pleasure and happiness, not by the universal moral law dictated by reason. To act in pursuit of happiness is arbitrary and subjective, and is no more moral than acting on the basis of greed, or selfishness. All three emanate from subjective, non-rational grounds. The danger of utilitarianism lies in its embracing of baser instincts, while rejecting the indispensable role of reason and freedom in our actions.

10. References and Further Reading

  • Anthropology from a Pragmatic Point of View, trans. Victor Lyle Dowden. Southern Illinois University Press, 1996.
  • The Conflict of the Faculties, trans. Mary Gregor. Lincoln: University of Nebraska Press, 1992.
  • Correspondence. ed. Arnulf Zweig. New York: Cambridge University Press, 1999.
  • Critique of Judgment, trans. Werner S. Pluhar. Indianapolis: Hackett, 1987.
  • Critique of Practical Reason, trans. Mary Gregor. New York: Cambridge University Press, 1997.
  • Critique of Pure Reason, trans. Werner Pluhar. Indianapolis: Hackett, 1996.
  • Groundwork of the Metaphysics of Morals. ed. Mary Gregor. New York: Cambridge University Press, 1998.
  • Kant's Latin Writings, Translations, Commentaries, and Notes, trans. Lewis White Beck in collaboration with Mary Gregor, Ralf Meerbote, John Reuscher. New York: Peter Lang, 1986
  • Kant: Philosophical Correspondence 1759-1799, ed. and trans. Arnulf Zweig. Chicago: Chicago University Press, 1967.
  • Logic, trans. Robert S. Hartman and Wolfgang Schwarz. New York: Dover Publications, 1974.
  • Metaphysical Foundations of Natural Science, trans. James Ellington. Indianapolis: Hackett, 1975.
  • The Metaphysics of Morals. trans. Mary Gregor. New York: Cambridge University Press, 1996.
  • Opus Postumum, ed. Eckart Forster, trans. Eckart Forster and Michael Rosen. Cambridge: Cambridge University Press, 1993.
  • Prolegomena to Any Future Metaphysics, trans. Gary Hatfield. New York: Cambridge University Press, 1997.
  • Religion within the Limits of Reason Alone. trans. T.M. Greene and H.H. Hudson. New York: Harper and Row, 1960.
  • Theoretical Philosophy, trans. David Walford and Ralf Meerbote. Cambridge: Cambridge University Press, 1992.
  • What Real Progress Has Metaphysics Made in Germany Since the Time of Leibniz and Wolff?(1804). trans. T. Humphrey. New York: Abaris, 1983 (Ak. XX).

Author Information

Matt McCormick
Email: mccormick@csus.edu
California State University, Sacramento
U. S. A.

1. Life and works

Immanuel Kant was born April 22, 1724 in Königsberg, near the southeastern shore of the Baltic Sea. Today Königsberg has been renamed Kaliningrad and is part of Russia. But during Kant's lifetime Königsberg was the capital of East Prussia, and its dominant language was German. Though geographically remote from the rest of Prussia and other German cities, Königsberg was then a major commercial center, an important military port, and a relatively cosmopolitan university town.[1]

Kant was born into an artisan family of modest means. His father was a master harness maker, and his mother was the daughter of a harness maker, though she was better educated than most women of her social class. Kant's family was never destitute, but his father's trade was in decline during Kant's youth and his parents at times had to rely on extended family for financial support.

Kant's parents were Pietist and he attended a Pietist school, the Collegium Fridericianum, from ages eight through fifteen. Pietism was an evangelical Lutheran movement that emphasized conversion, reliance on divine grace, the experience of religious emotions, and personal devotion involving regular Bible study, prayer, and introspection. Kant reacted strongly against the forced soul-searching to which he was subjected at the Collegium Fridericianum, in response to which he sought refuge in the Latin classics, which were central to the school's curriculum. Later the mature Kant's emphasis on reason and autonomy, rather than emotion and dependence on either authority or grace, may in part reflect his youthful reaction against Pietism. But although the young Kant loathed his Pietist schooling, he had deep respect and admiration for his parents, especially his mother, whose “genuine religiosity” he described as “not at all enthusiastic.” According to his biographer, Manfred Kuehn, Kant's parents probably influenced him much less through their Pietism than through their artisan values of “hard work, honesty, cleanliness, and independence,” which they taught him by example.[2]

Kant attended college at the University of Königsberg, known as the Albertina, where his early interest in classics was quickly superseded by philosophy, which all first year students studied and which encompassed mathematics and physics as well as logic, metaphysics, ethics, and natural law. Kant's philosophy professors exposed him to the approach of Christian Wolff (1679–1750), whose critical synthesis of the philosophy of G. W. Leibniz (1646–1716) was then very influential in German universities. But Kant was also exposed to a range of German and British critics of Wolff, and there were strong doses of Aristotelianism and Pietism represented in the philosophy faculty as well. Kant's favorite teacher was Martin Knutzen (1713–1751), a Pietist who was heavily influenced by both Wolff and the English philosopher John Locke (1632–1704). Knutzen introduced Kant to the work of Isaac Newton (1642–1727), and his influence is visible in Kant's first published work, Thoughts on the True Estimation of Living Forces (1747), which was a critical attempt to mediate a dispute in natural philosophy between Leibnizians and Newtonians over the proper measurement of force.

After college Kant spent six years as a private tutor to young children outside Königsberg. By this time both of his parents had died and Kant's finances were not yet secure enough for him to pursue an academic career. He finally returned to Königsberg in 1754 and began teaching at the Albertina the following year. For the next four decades Kant taught philosophy there, until his retirement from teaching in 1796 at the age of seventy-two.

Kant had a burst of publishing activity in the years after he returned from working as a private tutor. In 1754 and 1755 he published three scientific works — one of which, Universal Natural History and Theory of the Heavens (1755), was a major book in which, among other things, he developed what later became known as the nebular hypothesis about the formation of the solar system. Unfortunately, the printer went bankrupt and the book had little immediate impact. To secure qualifications for teaching at the university, Kant also wrote two Latin dissertations: the first, entitled Concise Outline of Some Reflections on Fire (1755), earned him the Magister degree; and the second, New Elucidation of the First Principles of Metaphysical Cognition (1755), entitled him to teach as an unsalaried lecturer. The following year he published another Latin work, The Employment in Natural Philosophy of Metaphysics Combined with Geometry, of Which Sample I Contains the Physical Monadology (1756), in hopes of succeeding Knutzen as associate professor of logic and metaphysics, though Kant failed to secure this position. Both the New Elucidation, which was Kant's first work concerned mainly with metaphysics, and the Physical Monadology further develop the position on the interaction of finite substances that he first outlined in Living Forces. Both works depart from Leibniz-Wolffian views, though not radically. The New Elucidation in particular shows the influence of Christian August Crusius (1715–1775), a German critic of Wolff.[3]

As an unsalaried lecturer at the Albertina Kant was paid directly by the students who attended his lectures, so he needed to teach an enormous amount and to attract many students in order to earn a living. Kant held this position from 1755 to 1770, during which period he would lecture an average of twenty hours per week on logic, metaphysics, and ethics, as well as mathematics, physics, and physical geography. In his lectures Kant used textbooks by Wolffian authors such as Alexander Gottlieb Baumgarten (1714–1762) and Georg Friedrich Meier (1718–1777), but he followed them loosely and used them to structure his own reflections, which drew on a wide range of ideas of contemporary interest. These ideas often stemmed from British sentimentalist philosophers such as David Hume (1711–1776) and Francis Hutcheson (1694–1747), some of whose texts were translated into German in the mid-1750's; and from the Swiss philosopher Jean-Jacques Rousseau (1712–1778), who published a flurry of works in the early 1760's. From early in his career Kant was a popular and successful lecturer. He also quickly developed a local reputation as a promising young intellectual and cut a dashing figure in Königsberg society.

After several years of relative quiet, Kant unleashed another burst of publications in 1762–1764, including five philosophical works. The False Subtlety of the Four Syllogistic Figures (1762) rehearses criticisms of Aristotelian logic that were developed by other German philosophers. The Only Possible Argument in Support of a Demonstration of the Existence of God (1762–3) is a major book in which Kant drew on his earlier work in Universal History and New Elucidation to develop an original argument for God's existence as a condition of the internal possibility of all things, while criticizing other arguments for God's existence. The book attracted several positive and some negative reviews. In 1762 Kant also submitted an essay entitled Inquiry Concerning the Distinctness of the Principles of Natural Theology and Morality to a prize competition by the Prussian Royal Academy, though Kant's submission took second prize to Moses Mendelssohn's winning essay (and was published with it in 1764). Kant's Prize Essay, as it is known, departs more significantly from Leibniz-Wolffian views than his earlier work and also contains his first extended discussion of moral philosophy in print. The Prize Essay draws on British sources to criticize German rationalism in two respects: first, drawing on Newton, Kant distinguishes between the methods of mathematics and philosophy; and second, drawing on Hutcheson, he claims that “an unanalysable feeling of the good” supplies the material content of our moral obligations, which cannot be demonstrated in a purely intellectual way from the formal principle of perfection alone (2:299).[4] These themes reappear in the Attempt to Introduce the Concept of Negative Magnitudes into Philosophy (1763), whose main thesis, however, is that the real opposition of conflicting forces, as in causal relations, is not reducible to the logical relation of contradiction, as Leibnizians held. In Negative Magnitudes Kant also argues that the morality of an action is a function of the internal forces that motivate one to act, rather than of the external (physical) actions or their consequences. Finally, Observations on the Feeling of the Beautiful and the Sublime (1764) deals mainly with alleged differences in the tastes of men and women and of people from different cultures. After it was published, Kant filled his own interleaved copy of this book with (often unrelated) handwritten remarks, many of which reflect the deep influence of Rousseau on his thinking about moral philosophy in the mid-1760's.

These works helped to secure Kant a broader reputation in Germany, but for the most part they were not strikingly original. Like other German philosophers at the time, Kant's early works are generally concerned with using insights from British empiricist authors to reform or broaden the German rationalist tradition without radically undermining its foundations. While some of his early works tend to emphasize rationalist ideas, others have a more empiricist emphasis. During this time Kant was striving to work out an independent position, but before the 1770's his views remained fluid.

In 1766 Kant published his first work concerned with the possibility of metaphysics, which later became a central topic of his mature philosophy. Dreams of a Spirit-Seer Elucidated by Dreams of Metaphysics, which he wrote soon after publishing a short Essay on Maladies of the Mind (1764), was occasioned by Kant's fascination with the Swedish visionary Emanuel Swedenborg (1688–1772), who claimed to have insight into a spirit world that enabled him to make a series of apparently miraculous predictions. In this curious work Kant satirically compares Swedenborg's spirit-visions to the belief of rationalist metaphysicians in an immaterial soul that survives death, and he concludes that philosophical knowledge of either is impossible because human reason is limited to experience. The skeptical tone of Dreams is tempered, however, by Kant's suggestion that “moral faith” nevertheless supports belief in an immaterial and immortal soul, even if it is not possible to attain metaphysical knowledge in this domain (2:373).

In 1770, at the age of forty-six, Kant was appointed to the chair in logic and metaphysics at the Albertina, after teaching for fifteen years as an unsalaried lecturer and working since 1766 as a sublibrarian to supplement his income. Kant was turned down for the same position in 1758. But later, as his reputation grew, he declined chairs in philosophy at Erlangen (1769) and Jena (1770) in hopes of obtaining one in Königsberg. After Kant was finally promoted, he gradually extended his repertoire of lectures to include anthropology (Kant's was the first such course in Germany and became very popular), rational theology, pedagogy, natural right, and even mineralogy and military fortifications. In order to inaugurate his new position, Kant also wrote one more Latin dissertation: Concerning the Form and Principles of the Sensible and Intelligible World (1770), which is known as the Inaugural Dissertation.

The Inaugural Dissertation departs more radically from both Wolffian rationalism and British sentimentalism than Kant's earlier work. Inspired by Crusius and the Swiss natural philosopher Johann Heinrich Lambert (1728–1777), Kant distinguishes between two fundamental powers of cognition, sensibility and understanding (intelligence), where the Leibniz-Wolffians regarded understanding (intellect) as the only fundamental power. Kant therefore rejects the rationalist view that sensibility is only a confused species of intellectual cognition, and he replaces this with his own view that sensibility is distinct from understanding and brings to perception its own subjective forms of space and time — a view that developed out of Kant's earlier criticism of Leibniz's relational view of space in Concerning the Ultimate Ground of the Differentiation of Directions in Space (1768). Moreover, as the title of the Inaugural Dissertation indicates, Kant argues that sensibility and understanding are directed at two different worlds: sensibility gives us access to the sensible world, while understanding enables us to grasp a distinct intelligible world. These two worlds are related in that what the understanding grasps in the intelligible world is the “paradigm” of “NOUMENAL PERFECTION,” which is “a common measure for all other things in so far as they are realities.” Considered theoretically, this intelligible paradigm of perfection is God; considered practically, it is “MORAL PERFECTION” (2:396). The Inaugural Dissertation thus develops a form of Platonism; and it rejects the view of British sentimentalists that moral judgments are based on feelings of pleasure or pain, since Kant now holds that moral judgments are based on pure understanding alone.

After 1770 Kant never surrendered the views that sensibility and understanding are distinct powers of cognition, that space and time are subjective forms of human sensibility, and that moral judgments are based on pure understanding (or reason) alone. But his embrace of Platonism in the Inaugural Dissertation was short-lived. He soon denied that our understanding is capable of insight into an intelligible world, which cleared the path toward his mature position in the Critique of Pure Reason (1781), according to which the understanding (like sensibility) supplies forms that structure our experience of the sensible world, to which human knowledge is limited, while the intelligible (or noumenal) world is strictly unknowable to us. Kant spent a decade working on the Critique of Pure Reason and published nothing else of significance between 1770 and 1781. But its publication marked the beginning of another burst of activity that produced Kant's most important and enduring works. Because early reviews of the Critique of Pure Reason were few and (in Kant's judgment) uncomprehending, he tried to clarify its main points in the much shorter Prolegomena to Any Future Metaphysics That Will Be Able to Come Forward as a Science (1783). Among the major books that rapidly followed are the Groundwork of the Metaphysics of Morals (1785), Kant's main work on the fundamental principle of morality; the Metaphysical Foundations of Natural Science (1786), his main work on natural philosophy in what scholars call his critical period (1781–1798); the second and substantially revised edition of the Critique of Pure Reason (1787); the Critique of Practical Reason (1788), a fuller discussion of topics in moral philosophy that builds on (and in some ways revises) the Groundwork; and the Critique of the Power of Judgment (1790), which deals with aesthetics and teleology. Kant also published a number of important essays in this period, including Idea for a Universal History With a Cosmopolitan Aim (1784) and Conjectural Beginning of Human History (1786), his main contributions to the philosophy of history; An Answer to the Question: What is Enlightenment? (1784), which broaches some of the key ideas of his later political essays; and What Does it Mean to Orient Oneself in Thinking? (1786), Kant's intervention in the pantheism controversy that raged in German intellectual circles after F. H. Jacobi (1743–1819) accused the recently deceased G. E. Lessing (1729–1781) of Spinozism.

With these works Kant secured international fame and came to dominate German philosophy in the late 1780's. But in 1790 he announced that the Critique of the Power of Judgment brought his critical enterprise to an end (5:170). By then K. L. Reinhold (1758–1823), whose Letters on the Kantian Philosophy (1786) popularized Kant's moral and religious ideas, had been installed (in 1787) in a chair devoted to Kantian philosophy at Jena, which was more centrally located than Königsberg and rapidly developing into the focal point of the next phase in German intellectual history. Reinhold soon began to criticize and move away from Kant's views. In 1794 his chair at Jena passed to J. G. Fichte, who had visited the master in Königsberg and whose first book, Attempt at a Critique of All Revelation (1792), was published anonymously and initially mistaken for a work by Kant himself. This catapulted Fichte to fame, but he too soon moved away from Kant and developed an original position quite at odds with Kant's, which Kant finally repudiated publicly in 1799 (12:370–371). Yet while German philosophy moved on to assess and respond to Kant's legacy, Kant himself continued publishing important works in the 1790's. Among these are Religion Within the Boundaries of Mere Reason (1793), which drew a censure from the Prussian King when Kant published the book after its second essay was rejected by the censor; The Conflict of the Faculties (1798), a collection of essays inspired by Kant's troubles with the censor and dealing with the relationship between the philosophical and theological faculties of the university; On the Common Saying: That May be Correct in Theory, But it is of No Use in Practice (1793), Toward Perpetual Peace (1795), and the Doctrine of Right, the first part of the Metaphysics of Morals (1797), Kant's main works in political philosophy; the Doctrine of Virtue, the second part of the Metaphysics of Morals (1797), a catalogue of duties that Kant had been planning for more than thirty years; and Anthropology From a Pragmatic Point of View (1798), based on Kant's anthropology lectures. Several other compilations of Kant's lecture notes from other courses were published later, but these were not prepared by Kant himself.

Kant retired from teaching in 1796. For nearly two decades he had lived a highly disciplined life focused primarily on completing his philosophical system, which began to take definite shape in his mind only in middle age. After retiring he came to believe that there was a gap in this system separating the metaphysical foundations of natural science from physics itself, and he set out to close this gap in a series of notes that postulate the existence of an ether or caloric matter. These notes, known as the Opus Postumum, remained unfinished and unpublished in Kant's lifetime, and scholars disagree on their significance and relation to his earlier work. It is clear, however, that these late notes show unmistakable signs of Kant's mental decline, which became tragically precipitous around 1800. Kant died February 12, 1804, just short of his eightieth birthday.

2. Kant's project in the Critique of Pure Reason

The main topic of the Critique of Pure Reason is the possibility of metaphysics, understood in a specific way. Kant defines metaphysics in terms of “the cognitions after which reason might strive independently of all experience,” and his goal in the book is to reach a “decision about the possibility or impossibility of a metaphysics in general, and the determination of its sources, as well as its extent and boundaries, all, however, from principles” (Axii. See also Bxiv; and 4:255–257). Thus metaphysics for Kant concerns a priori knowledge, or knowledge whose justification does not depend on experience; and he associates a priori knowledge with reason. The project of the Critique is to examine whether, how, and to what extent human reason is capable of a priori knowledge.

2.1 The crisis of the Enlightenment

To understand the project of the Critique better, let us consider the historical and intellectual context in which it was written.[5] Kant wrote the Critique toward the end of the Enlightenment, which was then in a state of crisis. Hindsight enables us to see that the 1780’s was a transitional decade in which the cultural balance shifted decisively away from the Enlightenment toward Romanticism, but of course Kant did not have the benefit of such hindsight.

The Enlightenment was a reaction to the rise and successes of modern science in the sixteenth and seventeenth centuries. The spectacular achievement of Newton in particular engendered widespread confidence and optimism about the power of human reason to control nature and to improve human life. One effect of this new confidence in reason was that traditional authorities were increasingly questioned. For why should we need political or religious authorities to tell us how to live or what to believe, if each of us has the capacity to figure these things out for ourselves? Kant expresses this Enlightenment commitment to the sovereignty of reason in the Critique:

Our age is the age of criticism, to which everything must submit. Religion through its holiness and legislation through its majesty commonly seek to exempt themselves from it. But in this way they excite a just suspicion against themselves, and cannot lay claim to that unfeigned respect that reason grants only to that which has been able to withstand its free and public examination (Axi).

Enlightenment is about thinking for oneself rather than letting others think for you, according to What is Enlightenment? (8:35). In this essay, Kant also expresses the Enlightenment faith in the inevitability of progress. A few independent thinkers will gradually inspire a broader cultural movement, which ultimately will lead to greater freedom of action and governmental reform. A culture of enlightenment is “almost inevitable” if only there is “freedom to make public use of one's reason in all matters” (8:36).

The problem is that to some it seemed unclear whether progress would in fact ensue if reason enjoyed full sovereignty over traditional authorities; or whether unaided reasoning would instead lead straight to materialism, fatalism, atheism, skepticism (Bxxxiv), or even libertinism and authoritarianism (8:146). The Enlightenment commitment to the sovereignty of reason was tied to the expectation that it would not lead to any of these consequences but instead would support certain key beliefs that tradition had always sanctioned. Crucially, these included belief in God, the soul, freedom, and the compatibility of science with morality and religion. Although a few intellectuals rejected some or all of these beliefs, the general spirit of the Enlightenment was not so radical. The Enlightenment was about replacing traditional authorities with the authority of individual human reason, but it was not about overturning traditional moral and religious beliefs.

Yet the original inspiration for the Enlightenment was the new physics, which was mechanistic. If nature is entirely governed by mechanistic, causal laws, then it may seem that there is no room for freedom, a soul, or anything but matter in motion. This threatened the traditional view that morality requires freedom. We must be free in order to choose what is right over what is wrong, because otherwise we cannot be held responsible. It also threatened the traditional religious belief in a soul that can survive death or be resurrected in an afterlife. So modern science, the pride of the Enlightenment, the source of its optimism about the powers of human reason, threatened to undermine traditional moral and religious beliefs that free rational thought was expected to support. This was the main intellectual crisis of the Enlightenment.

The Critique of Pure Reason is Kant's response to this crisis. Its main topic is metaphysics because, for Kant, metaphysics is the domain of reason – it is “the inventory of all we possess through pure reason, ordered systematically” (Axx) — and the authority of reason was in question. Kant's main goal is to show that a critique of reason by reason itself, unaided and unrestrained by traditional authorities, establishes a secure and consistent basis for both Newtonian science and traditional morality and religion. In other words, free rational inquiry adequately supports all of these essential human interests and shows them to be mutually consistent. So reason deserves the sovereignty attributed to it by the Enlightenment.

2.2 Kant's Copernican revolution in philosophy

To see how Kant attempts to achieve this goal in the Critique, it helps to reflect on his grounds for rejecting the Platonism of the Inaugural Dissertation. In a way the Inaugural Dissertation also tries to reconcile Newtonian science with traditional morality and religion, but its strategy is different from that of the Critique. According to the Inaugural Dissertation, Newtonian science is true of the sensible world, to which sensibility gives us access; and the understanding grasps principles of divine and moral perfection in a distinct intelligible world, which are paradigms for measuring everything in the sensible world. So on this view our knowledge of the intelligible world is a priori because it does not depend on sensibility, and this a priori knowledge furnishes principles for judging the sensible world because in some way the sensible world itself conforms to or imitates the intelligible world.

Soon after writing the Inaugural Dissertation, however, Kant expressed doubts about this view. As he explained in a February 21, 1772 letter to his friend and former student, Marcus Herz:

In my dissertation I was content to explain the nature of intellectual representations in a merely negative way, namely, to state that they were not modifications of the soul brought about by the object. However, I silently passed over the further question of how a representation that refers to an object without being in any way affected by it can be possible…. [B]y what means are these [intellectual representations] given to us, if not by the way in which they affect us? And if such intellectual representations depend on our inner activity, whence comes the agreement that they are supposed to have with objects — objects that are nevertheless not possibly produced thereby?…[A]s to how my understanding may form for itself concepts of things completely a priori, with which concepts the things must necessarily agree, and as to how my understanding may formulate real principles concerning the possibility of such concepts, with which principles experience must be in exact agreement and which nevertheless are independent of experience — this question, of how the faculty of understanding achieves this conformity with the things themselves, is still left in a state of obscurity. (10:130–131)

Here Kant entertains doubts about how a priori knowledge of an intelligible world would be possible. The position of the Inaugural Dissertation is that the intelligible world is independent of the human understanding and of the sensible world, both of which (in different ways) conform to the intelligible world. But, leaving aside questions about what it means for the sensible world to conform to an intelligible world, how is it possible for the human understanding to conform to or grasp an intelligible world? If the intelligible world is independent of our understanding, then it seems that we could grasp it only if we are passively affected by it in some way. But for Kant sensibility is our passive or receptive capacity to be affected by objects that are independent of us (2:392, A51/B75). So the only way we could grasp an intelligible world that is independent of us is through sensibility, which means that our knowledge of it could not be a priori. The pure understanding alone could at best enable us to form representations of an intelligible world. But since these intellectual representations would entirely “depend on our inner activity,” as Kant says to Herz, we have no good reason to believe that they conform to an independent intelligible world. Such a priori intellectual representations could well be figments of the brain that do not correspond to anything independent of the human mind. In any case, it is completely mysterious how there might come to be a correspondence between purely intellectual representations and an independent intelligible world.

Kant's strategy in the Critique is similar to that of the Inaugural Dissertation in that both works attempt to reconcile modern science with traditional morality and religion by relegating them to distinct sensible and intelligible worlds, respectively. But the Critique gives a far more modest and yet revolutionary account of a priori knowledge. As Kant's letter to Herz suggests, the main problem with his view in the Inaugural Dissertation is that it tries to explain the possibility of a priori knowledge about a world that is entirely independent of the human mind. This turned out to be a dead end, and Kant never again maintained that we can have a priori knowledge about an intelligible world precisely because such a world would be entirely independent of us. However, Kant's revolutionary position in the Critique is that we can have a priori knowledge about the general structure of the sensible world because it is not entirely independent of the human mind. The sensible world, or the world of appearances, is constructed by the human mind from a combination of sensory matter that we receive passively and a priori forms that are supplied by our cognitive faculties. We can have a priori knowledge only about aspects of the sensible world that reflect the a priori forms supplied by our cognitive faculties. In Kant's words, “we can cognize of things a priori only what we ourselves have put into them” (Bxviii). So according to the Critique, a priori knowledge is possible only if and to the extent that the sensible world itself depends on the way the human mind structures its experience.

Kant characterizes this new constructivist view of experience in the Critique through an analogy with the revolution wrought by Copernicus in astronomy:

Up to now it has been assumed that all our cognition must conform to the objects; but all attempts to find out something about them a priori through concepts that would extend our cognition have, on this presupposition, come to nothing. Hence let us once try whether we do not get farther with the problems of metaphysics by assuming that the objects must conform to our cognition, which would agree better with the requested possibility of an a priori cognition of them, which is to establish something about objects before they are given to us. This would be just like the first thoughts of Copernicus, who, when he did not make good progress in the explanation of the celestial motions if he assumed that the entire celestial host revolves around the observer, tried to see if he might not have greater success if he made the observer revolve and left the stars at rest. Now in metaphysics we can try in a similar way regarding the intuition of objects. If intuition has to conform to the constitution of the objects, then I do not see how we can know anything of them a priori; but if the object (as an object of the senses) conforms to the constitution of our faculty of intuition, then I can very well represent this possibility to myself. Yet because I cannot stop with these intuitions, if they are to become cognitions, but must refer them as representations to something as their object and determine this object through them, I can assume either that the concepts through which I bring about this determination also conform to the objects, and then I am once again in the same difficulty about how I could know anything about them a priori, or else I assume that the objects, or what is the same thing, the experience in which alone they can be cognized (as given objects) conforms to those concepts, in which case I immediately see an easier way out of the difficulty, since experience itself is a kind of cognition requiring the understanding, whose rule I have to presuppose in myself before any object is given to me, hence a priori, which rule is expressed in concepts a priori, to which all objects of experience must therefore necessarily conform, and with which they must agree. (Bxvi-xviii)

As this passage suggests, what Kant has changed in the Critique is primarily his view about the role and powers of the understanding, since he already held in the Inaugural Dissertation that sensibility contributes the forms of space and time — which he calls pure (or a priori) intuitions (2:397) — to our cognition of the sensible world. But the Critique claims that pure understanding too, rather than giving us insight into an intelligible world, is limited to providing forms — which he calls pure or a priori concepts — that structure our cognition of the sensible world. So now both sensibility and understanding work together to construct cognition of the sensible world, which therefore conforms to the a priori forms that are supplied by our cognitive faculties: the a priori intuitions of sensibility and the a priori concepts of the understanding. This account is analogous to the heliocentric revolution of Copernicus in astronomy because both require contributions from the observer to be factored into explanations of phenomena, although neither reduces phenomena to the contributions of observers alone.[6] The way celestial phenomena appear to us on earth, according to Copernicus, is affected by both the motions of celestial bodies and the motion of the earth, which is not a stationary body around which everything else revolves. For Kant, analogously, the phenomena of human experience depend on both the sensory data that we receive passively through sensibility and the way our mind actively processes this data according to its own a priori rules. These rules supply the general framework in which the sensible world and all the objects (or phenomena) in it appear to us. So the sensible world and its phenomena are not entirely independent of the human mind, which contributes its basic structure.

How does Kant's Copernican revolution in philosophy improve on the strategy of the Inaugural Dissertation for reconciling modern science with traditional morality and religion? First, it gives Kant a new and ingenious way of placing modern science on an a priori foundation. He is now in a position to argue that we can have a priori knowledge about the basic laws of modern science because those laws reflect the human mind's contribution to structuring our experience. In other words, the sensible world necessarily conforms to certain fundamental laws — such as that every event has a cause — because the human mind constructs it according to those laws. Moreover, we can identify those laws by reflecting on the conditions of possible experience, which reveals that it would be impossible for us to experience a world in which, for example, any given event fails to have a cause. From this Kant concludes that metaphysics is indeed possible in the sense that we can have a priori knowledge that the entire sensible world — not just our actual experience, but any possible human experience — necessarily conforms to certain laws. Kant calls this immanent metaphysics or the metaphysics of experience, because it deals with the essential principles that are immanent to human experience.

But, second, if “we can cognize of things a priori only what we ourselves have put into them,” then we cannot have a priori knowledge about things whose existence and nature are entirely independent of the human mind, which Kant calls things in themselves (Bxviii). In his words: “[F]rom this deduction of our faculty of cognizing a priori [...] there emerges a very strange result [...], namely that with this faculty we can never get beyond the boundaries of possible experience, [...and] that such cognition reaches appearances only, leaving the thing in itself as something actual for itself but uncognized by us” (Bxix-xx). That is, Kant's constructivist foundation for scientific knowledge restricts science to the realm of appearances and implies that a priori knowledge of things in themselves that transcend possible human experience — or transcendent metaphysics — is impossible. In the Critique Kant thus rejects the insight into an intelligible world that he defended in the Inaugural Dissertation, and he now claims that rejecting knowledge about things in themselves is necessary for reconciling science with traditional morality and religion. This is because he claims that belief in God, freedom, and immortality have a strictly moral basis, and yet adopting these beliefs on moral grounds would be unjustified if we could know that they were false. “Thus,” Kant says, “I had to deny knowledge in order to make room for faith” (Bxxx). Restricting knowledge to appearances and relegating God and the soul to an unknowable realm of things in themselves guarantees that it is impossible to disprove claims about God and the freedom or immortality of the soul, which moral arguments may therefore justify us in believing. Moreover, the determinism of modern science no longer threatens the freedom required by traditional morality, because science and therefore determinism apply only to appearances, and there is room for freedom in the realm of things in themselves, where the self or soul is located. We cannot know (theoretically) that we are free, because we cannot know anything about things in themselves. But there are especially strong moral grounds for the belief in human freedom, which acts as “the keystone” supporting other morally grounded beliefs (5:3–4). In this way, Kant replaces transcendent metaphysics with a new practical science that he calls the metaphysics of morals. It thus turns out that two kinds of metaphysics are possible: the metaphysics of experience (or nature) and the metaphysics of morals, both of which depend on Kant's Copernican revolution in philosophy.

3. Transcendental idealism

Perhaps the central and most controversial thesis of the Critique of Pure Reason is that human beings experience only appearances, not things in themselves; and that space and time are only subjective forms of human intuition that would not subsist in themselves if one were to abstract from all subjective conditions of human intuition. Kant calls this thesis transcendental idealism.[7] One of his best summaries of it is arguably the following:

We have therefore wanted to say that all our intuition is nothing but the representation of appearance; that the things that we intuit are not in themselves what we intuit them to be, nor are their relations so constituted in themselves as they appear to us; and that if we remove our own subject or even only the subjective constitution of the senses in general, then all constitution, all relations of objects in space and time, indeed space and time themselves would disappear, and as appearances they cannot exist in themselves, but only in us. What may be the case with objects in themselves and abstracted from all this receptivity of our sensibility remains entirely unknown to us. We are acquainted with nothing except our way of perceiving them, which is peculiar to us, and which therefore does not necessarily pertain to every being, though to be sure it pertains to every human being. We are concerned solely with this. Space and time are its pure forms, sensation in general its matter. We can cognize only the former a priori, i.e., prior to all actual perception, and they are therefore called pure intuition; the latter, however, is that in our cognition that is responsible for its being called a posteriori cognition, i.e., empirical intuition. The former adheres to our sensibility absolutely necessarily, whatever sort of sensations we may have; the latter can be very different. (A42/B59–60)[8]

Kant introduces transcendental idealism in the part of the Critique called the Transcendental Aesthetic, and scholars generally agree that for Kant transcendental idealism encompasses at least the following claims:

  • In some sense, human beings experience only appearances, not things in themselves.
  • Space and time are not things in themselves, or determinations of things in themselves that would remain if one abstracted from all subjective conditions of human intuition. [Kant labels this conclusion a) at A26/B42 and again at A32–33/B49. It is at least a crucial part of what he means by calling space and time transcendentally ideal (A28/B44, A35–36/B52)].
  • Space and time are nothing other than the subjective forms of human sensible intuition. [Kant labels this conclusion b) at A26/B42 and again at A33/B49–50].
  • Space and time are empirically real, which means that “everything that can come before us externally as an object” is in both space and time, and that our internal intuitions of ourselves are in time (A28/B44, A34–35/B51–51).

But scholars disagree widely on how to interpret these claims, and there is no such thing as the standard interpretation of Kant's transcendental idealism. Two general types of interpretation have been especially influential, however. This section provides an overview of these two interpretations, although it should be emphasized that much important scholarship on transcendental idealism does not fall neatly into either of these two camps.

3.1 The two-objects interpretation

The two-objects reading is the traditional interpretation of Kant's transcendental idealism. It goes back to the earliest review of the Critique — the so-called Göttingen review by Christian Garve (1742–1798) and J. G. Feder (1740–1821)[9] — and it was the dominant way of interpreting Kant's transcendental idealism during his own lifetime. It has been a live interpretive option since then and remains so today, although it no longer enjoys the dominance that it once did.[10]

According to the two-objects interpretation, transcendental idealism is essentially a metaphysical thesis that distinguishes between two classes of objects: appearances and things in themselves. Another name for this view is the two-worlds interpretation, since it can also be expressed by saying that transcendental idealism essentially distinguishes between a world of appearances and another world of things in themselves.

Things in themselves, on this interpretation, are absolutely real in the sense that they would exist and have whatever properties they have even if no human beings were around to perceive them. Appearances, on the other hand, are not absolutely real in that sense, because their existence and properties depend on human perceivers. Moreover, whenever appearances do exist, in some sense they exist in the mind of human perceivers. So appearances are mental entities or mental representations. This, coupled with the claim that we experience only appearances, makes transcendental idealism a form of phenomenalism on this interpretation, because it reduces the objects of experience to mental representations. All of our experiences – all of our perceptions of objects and events in space, even those objects and events themselves, and all non-spatial but still temporal thoughts and feelings – fall into the class of appearances that exist in the mind of human perceivers. These appearances cut us off entirely from the reality of things in themselves, which are non-spatial and non-temporal. Yet Kant's theory, on this interpretation, nevertheless requires that things in themselves exist, because they must transmit to us the sensory data from which we construct appearances. In principle we cannot know how things in themselves affect our senses, because our experience and knowledge is limited to the world of appearances constructed by and in the mind. Things in themselves are therefore a sort of theoretical posit, whose existence and role are required by the theory but are not directly verifiable.

The main problems with the two-objects interpretation are philosophical. Most readers of Kant who have interpreted his transcendental idealism in this way have been — often very — critical of it, for reasons such as the following:

First, at best Kant is walking a fine line in claiming on the one hand that we can have no knowledge about things in themselves, but on the other hand that we know that things in themselves exist, that they affect our senses, and that they are non-spatial and non-temporal. At worst his theory depends on contradictory claims about what we can and cannot know about things in themselves. This objection was influentially articulated by Jacobi, when he complained that “without that presupposition [of things in themselves] I could not enter into the system, but with it I could not stay within it” (Jacobi 1787, 336).

Second, even if that problem is surmounted, it has seemed to many that Kant's theory, interpreted in this way, implies a radical form of skepticism that traps each of us within the contents of our own mind and cuts us off from reality. Some versions of this objection proceed from premises that Kant rejects. One version maintains that things in themselves are real while appearances are not, and hence that on Kant's view we cannot have experience or knowledge of reality. But Kant denies that appearances are unreal: they are just as real as things in themselves but are in a different metaphysical class. Another version claims that truth always involves a correspondence between mental representations and things in themselves, from which it would follow that on Kant's view it is impossible for us to have true beliefs about the world. But just as Kant denies that things in themselves are the only (or privileged) reality, he also denies that correspondence with things in themselves is the only kind of truth. Empirical judgments are true just in case they correspond with their empirical objects in accordance with the a priori principles that structure all possible human experience. But the fact that Kant can appeal in this way to an objective criterion of empirical truth that is internal to our experience has not been enough to convince some critics that Kant is innocent of an unacceptable form of skepticism, mainly because of his insistence on our irreparable ignorance about things in themselves.

Third and finally, Kant's denial that things in themselves are spatial or temporal has struck many of his readers as incoherent. The role of things in themselves, on the two-object interpretation, is to affect our senses and thereby to provide the sensory data from which our cognitive faculties construct appearances within the framework of our a priori intuitions of space and time and a priori concepts such as causality. But if there is no space, time, change, or causation in the realm of things in themselves, then how can things in themselves affect us? Transcendental affection seems to involve a causal relation between things in themselves and our sensibility. If this is simply the way we unavoidably think about transcendental affection, because we can give positive content to this thought only by employing the concept of a cause, while it is nevertheless strictly false that things in themselves affect us causally, then it seems not only that we are ignorant of how things in themselves really affect us. It seems, rather, to be incoherent that things in themselves could affect us at all if they are not in space or time.

3.2 The two-aspects interpretation

The two-aspects reading attempts to interpret Kant's transcendental idealism in a way that enables it to be defended against at least some of these objections. On this view, transcendental idealism does not distinguish between two classes of objects but rather between two different aspects of one and the same class of objects. For this reason it is also called the one-world interpretation, since it holds that there is only one world in Kant's ontology, and that at least some objects in that world have two different aspects: one aspect that appears to us, and another aspect that does not appear to us. That is, appearances are aspects of the same objects that also exist in themselves. So, on this reading, appearances are not mental representations, and transcendental idealism is not a form of phenomenalism.[11]

There are at least two main versions of the two-aspects theory. One version treats transcendental idealism as a metaphysical theory according to which objects have two aspects in the sense that they have two sets of properties: one set of relational properties that appear to us and are spatial and temporal, and another set of intrinsic properties that do not appear to us and are not spatial or temporal (Langton 1998). This property-dualist interpretation faces epistemological objections similar to those faced by the two-objects interpretation, because we are in no better position to acquire knowledge about properties that do not appear to us than we are to acquire knowledge about objects that do not appear to us. Moreover, this interpretation also seems to imply that things in themselves are spatial and temporal, since appearances have spatial and temporal properties, and on this view appearances are the same objects as things in themselves. But Kant explicitly denies that space and time are properties of things in themselves.

A second version of the two-aspects theory departs more radically from the traditional two-objects interpretation by denying that transcendental idealism is at bottom a metaphysical theory. Instead, it interprets transcendental idealism as a fundamentally epistemological theory that distinguishes between two standpoints on the objects of experience: the human standpoint, from which objects are viewed relative to epistemic conditions that are peculiar to human cognitive faculties (namely, the a priori forms of our sensible intuition); and the standpoint of an intuitive intellect, from which the same objects could be known in themselves and independently of any epistemic conditions (Allison 2004). Human beings cannot really take up the latter standpoint but can form only an empty concept of things as they exist in themselves by abstracting from all the content of our experience and leaving only the purely formal thought of an object in general. So transcendental idealism, on this interpretation, is essentially the thesis that we are limited to the human standpoint, and the concept of a thing in itself plays the role of enabling us to chart the boundaries of the human standpoint by stepping beyond them in abstract (but empty) thought.

One criticism of this epistemological version of the two-aspects theory is that it avoids the objections to other interpretations by attributing to Kant a more limited project than the text of the Critique warrants. There are passages that support this reading.[12] But there are also many passages in both editions of the Critique in which Kant describes appearances as representations in the mind and in which his distinction between appearances and things in themselves is given not only epistemological but metaphysical significance.[13] It is unclear whether all of these texts admit of a single, consistent interpretation.

4. The transcendental deduction

The transcendental deduction is the central argument of the Critique of Pure Reason and one of the most complex and difficult texts in the history of philosophy. Given its complexity, there are naturally many different ways of interpreting the deduction.[14] This brief overview provides one perspective on some of its main ideas.

The transcendental deduction occurs in the part of the Critique called the Analytic of Concepts, which deals with the a priori concepts that, on Kant's view, our understanding uses to construct experience together with the a priori forms of our sensible intuition (space and time), which he discussed in the Transcendental Aesthetic. Kant calls these a priori concepts “categories,” and he argues elsewhere (in the so-called metaphysical deduction) that they include such concepts as substance and cause. The goal of the transcendental deduction is to show that we have a priori concepts or categories that are objectively valid, or that apply necessarily to all objects in the world that we experience. To show this, Kant argues that the categories are necessary conditions of experience, or that we could not have experience without the categories. In Kant's words:

[T]he objective validity of the categories, as a priori concepts, rests on the fact that through them alone is experience possible (as far as the form of thinking is concerned). For they then are related necessarily and a priori to objects of experience, since only by means of them can any object of experience be thought at all.

The transcendental deduction of all a priori concepts therefore has a principle toward which the entire investigation must be directed, namely this: that they must be recognized as a priori conditions of the possibility of experiences (whether of the intuition that is encountered in them, or of the thinking). Concepts that supply the objective ground of the possibility of experience are necessary just for that reason. (A93–94/B126)

The strategy Kant employs to argue that the categories are conditions of experience is the main source of both the obscurity and the ingenuity of the transcendental deduction. His strategy is to argue that the categories are necessary specifically for self-consciousness, for which Kant often uses the Leibnizian term “apperception.”

4.1 Self-consciousness

One way to approach Kant's argument is to contrast his view of self-consciousness with two alternative views that he rejects. Each of these views, both Kant's and those he rejects, can be seen as offering competing answers the question: what is the source of our sense of an ongoing and invariable self that persists throughout all the changes in our experience?

The first answer to this question that Kant rejects is that self-consciousness arises from some particular content being present in each of one’s representations. This material conception of self-consciousness, as we may call it, is loosely suggested by Locke’s account of personal identity. According to Locke, “it being the same consciousness that makes a Man be himself to himself, personal Identity depends on that only, whether it be annexed only to one individual Substance, or can be continued in a succession of several Substances” (Essay 2.27.10). What Locke calls “the same consciousness” may be understood as some representational content that is always present in my experience and that both identifies any experience as mine and gives me a sense of a continuous self by virtue of its continual presence in my experience. One problem with this view, Kant believes, is that there is no such representational content that is invariably present in experience, so the sense of an ongoing self cannot possibly arise from that non-existent content (what Locke calls “consciousness”) being present in each of one's representations. In Kant's words, self-consciousness “does not yet come about by my accompanying each representation with consciousness, but rather by my adding one representation to the other and being conscious of their synthesis. Therefore it is only because I can combine a manifold of given representations in one consciousness that it is possible for me to represent the identity of the consciousness in these representations” (B133). Here Kant claims, against the Lockean view, that self-consciousness arises from combining (or synthesizing) representations with one another regardless of their content. In short, Kant has a formal conception of self-consciousness rather than a material one. Since no particular content of my experience is invariable, self-consciousness must derive from my experience having an invariable form or structure, and consciousness of the identity of myself through all of my changing experiences must consist in awareness of the formal unity and law-governed regularity of my experience. The continuous form of my experience is the necessary correlate for my sense of a continuous self.

There are at least two possible versions of the formal conception of self-consciousness: a realist and an idealist version. On the realist version, nature itself is law-governed and we become self-conscious by attending to its law-governed regularities, which also makes this an empiricist view of self-consciousness. The idea of an identical self that persists throughout all of our experience, on this view, arises from the law-governed regularity of nature, and our representations exhibit order and regularity because reality itself is ordered and regular. But Kant rejects this view and embraces a conception of self-consciousness that is both formal and idealist. According to Kant, the formal structure of our experience, its unity and law-governed regularity, is an achievement of our cognitive faculties rather than a property of reality in itself. Our experience has a constant form because our mind constructs experience in a law-governed way. So self-consciousness, for Kant, consists in awareness of the mind's law-governed activity of synthesizing or combining sensible data to construct a unified experience. As he expresses it, “this unity of consciousness would be impossible if in the cognition of the manifold the mind could not become conscious of the identity of the function by means of which this manifold is synthetically combined into one cognition” (A108).

Kant argues for this formal idealist conception of self-consciousness, and against the formal realist view, on the grounds that “we can represent nothing as combined in the object without having previously combined it ourselves” (B130). In other words, even if reality in itself were law-governed, its laws could not simply migrate over to our mind or imprint themselves on us while our mind is entirely passive. We must exercise an active capacity to represent the world as combined or ordered in a law-governed way, because otherwise we could not represent the world as law-governed even if it were law-governed in itself. Moreover, this capacity to represent the world as law-governed must be a priori because it is a condition of self-consciousness, and we would already have to be self-conscious in order to learn from our experience that there are law-governed regularities in the world. So it is necessary for self-consciousness that we exercise an a priori capacity to represent the world as law-governed. But this would also be sufficient for self-consciousness if we could exercise our a priori capacity to represent the world as law-governed even if reality in itself were not law-governed. In that case, the realist and empiricist conception of self-consciousness would be false, and the formal idealist view would be true.

Kant's confidence that no empiricist account could possibly explain self-consciousness may be based on his assumption that the sense of self each of us has, the thought of oneself as identical throughout all of one's changing experiences, involves necessity and universality, which on his view are the hallmarks of the a priori. This assumption is reflected in what we may call Kant's principle of apperception: “The I think must be able to accompany all my representations; for otherwise something would be represented in me that could not be thought at all, which is as much as to say that the representation would either be impossible or else at least would be nothing for me” (B131–132).[15] Notice the claims about necessity and universality embodied in the words “must” and “all” here. Kant is saying that for a representation to count as mine, it must necessarily be accessible to conscious awareness in some (perhaps indirect) way: I must be able to accompany it with “I think....” All of my representations must be accessible to consciousness in this way (but they need not actually be conscious), because again that is simply what makes a representation count as mine. Self-consciousness for Kant therefore involves a priori knowledge about the necessary and universal truth expressed in this principle of apperception, and a priori knowledge cannot be based on experience.

Recently it has been argued that Kant developed this thread of his argument in the transcendental deduction after reading Johann Nicolaus Tetens (1736–1807) rather than simply through a direct encounter with Locke's texts (Tetens 1777, Kitcher 2011). On the subject of self-consciousness, Tetens was a follower of Locke and also engaged with Hume's arguments for rejecting a continuing self. So Kant's actual opponents in the deduction may have been Lockean and Humean positions as represented by Tetens, as well as rationalist views that Kant would have encountered directly in texts by Leibniz, Wolff, and some of their followers.

4.2 Objectivity and judgment

On the basis of this formal idealist conception of self-consciousness, Kant's argument (at least one central thread of it) moves through two more conditions of self-consciousness in order to establish the objective validity of the categories. The next condition is that self-consciousness requires me to represent an objective world distinct from my subjective representations - that is, distinct from my thoughts about and sensations of that objective world. Kant uses this connection between self-consciousness and objectivity to insert the categories into his argument.

In order to be self-conscious, I cannot be wholly absorbed in the contents of my perceptions but must distinguish myself from the rest of the world. But if self-consciousness is an achievement of the mind, then how does the mind achieve this sense that there is a distinction between the I that perceives and the contents of its perceptions? According to Kant, the mind achieves this by distinguishing representations that necessarily belong together from representations that are not necessarily connected but are merely associated in a contingent way. Consider Kant's example of the perception of a house (B162). Imagine a house that is too large to fit into your visual field from your vantage point near its front door. Now imagine that you walk around the house, successively perceiving each of its sides. Eventually you perceive the entire house, but not all at once, and you judge that each of your representations of the sides of the house necessarily belong together (as sides of one house) and that anyone who denied this would be mistaken. But now imagine that you grew up in this house and associate a feeling of nostalgia with it. You would not judge that representations of this house are necessarily connected with feelings of nostalgia. That is, you would not think that other people seeing the house for the first time would be mistaken if they denied that it is connected with nostalgia, because you recognize that this house is connected with nostalgia for you but not necessarily for everyone. Yet you distinguish this merely subjective connection from the objective connection between sides of the house, which is objective because the sides of the house necessarily belong together “in the object,” because this connection holds for everyone universally, and because it is possible to be mistaken about it. The point here is not that we must successfully identify which representations necessarily belong together and which are merely associated contingently, but rather that to be self-conscious we must at least make this general distinction between objective and merely subjective connections of representations.

At this point (at least in the second edition text) Kant introduces the key claim that judgment is what enables us to distinguish objective connections of representations that necessarily belong together from merely subjective and contingent associations: “[A] judgment is nothing other than the way to bring given cognitions to the objective unity of apperception. That is the aim of the copula is in them: to distinguish the objective unity of given representations from the subjective. For this word designates the relation of the representations to the original apperception and its necessary unity” (B141–142). Kant is speaking here about the mental act of judging that results in the formation of a judgment. Judging is an act of what Kant calls synthesis, which he defines as “the action of putting different representations together with each other and comprehending their manifoldness in one cognition” (A77/B103). In other words, to synthesize is in general to combine several representations into a single (more) complex representation, and to judge is specifically to combine concepts into a judgment — that is, to join a subject concept to a predicate concept by means of the copula, as in “the body is heavy” or “the house is four-sided.” Judgments need not be true, of course, but they always have a truth value (true or false) because they make claims to objective validity. When I say, by contrast, that “If I carry a body, I feel a pressure of weight,” or that “if I see this house, I feel nostalgia,” I am not making a judgment about the object (the body or the house) but rather I am expressing a subjective association that may apply only to me (B142).[16]

Kant's reference to the necessary unity of apperception or self-consciousness in the quotation above means (at least) that the action of judging is the way our mind achieves self-consciousness. We must represent an objective world in order to distinguish ourselves from it, and we represent an objective world by judging that some representations necessarily belong together. Moreover, recall from 4.1 that, for Kant, we must have an a priori capacity to represent the world as law-governed, because “we can represent nothing as combined (or connected) in the object without having previously combined it ourselves” (B130). It follows that objective connections in the world cannot simply imprint themselves on our mind. Rather, experience of an objective world must be constructed by exercising an a priori capacity to judge, which Kant calls the faculty of understanding (A80–81/B106). The understanding constructs experience by providing the a priori rules, or the framework of necessary laws, in accordance with which we judge representations to be objective. These rules are the pure concepts of the understanding or categories, which are therefore conditions of self-consciousness, since they are rules for judging about an objective world, and self-consciousness requires that we distinguish ourselves from an objective world.

Kant identifies the categories in what he calls the metaphysical deduction, which precedes the transcendental deduction.[17] Very briefly, since the categories are a priori rules for judging, Kant argues that an exhaustive table of categories can be derived from a table of the basic logical forms of judgments. For example, according to Kant the logical form of the judgment that “the body is heavy” would be singular, affirmative, categorical, and assertoric. But since categories are not mere logical functions but instead are rules for making judgments about objects or an objective world, Kant arrives at his table of categories by considering how each logical function would structure judgments about objects (within our spatio-temporal forms of intuition). For example, he claims that categorical judgments express a logical relation between subject and predicate that corresponds to the ontological relation between substance and accident; and the logical form of a hypothetical judgment expresses a relation that corresponds to cause and effect. Taken together with this argument, then, the transcendental deduction argues that we become self-conscious by representing an objective world of substances that interact according to causal laws.

4.3 The law-giver of nature

The final condition of self-consciousness that Kant adds to the preceding conditions is that our understanding must cooperate with sensibility to construct one, unbounded, and unified space-time to which all of our representations may be related.

To see why this further condition is required, consider that so far we have seen why Kant holds that we must represent an objective world in order to be self-conscious, but we could represent an objective world even if it were not possible to relate all of our representations to this objective world. For all that has been said so far, we might still have unruly representations that we cannot relate in any way to the objective framework of our experience. On Kant's view, this would be a problem because, as we have seen, he holds that self-consciousness involves universality and necessity: according to his principle of apperception, “the I think must be able to accompany all my representations” (B131). Yet if, on the one hand, I had representations that I could not relate in some way to an objective world, then I could not accompany those representations with “I think” or recognize them as my representations, because I can say “I think…” about any given representation only by relating it to an objective world, according to the argument just discussed. So I must be able to relate any given representation to an objective world in order for it to count as mine. On the other hand, self-consciousness would also be impossible if I represented multiple objective worlds, even if I could relate all of my representations to some objective world or other. In that case, I could not become conscious of an identical self that has, say, representation 1 in space-time A and representation 2 in space-time B. It may be possible to imagine disjointed spaces and times, but it is not possible to represent them as objectively real. So self-consciousness requires that I can relate all of my representations to a single objective world.

The reason why I must represent this one objective world by means of a unified and unbounded space-time is that, as Kant argued in the Transcendental Aesthetic, space and time are the pure forms of human intuition. If we had different forms of intuition, then our experience would still have to constitute a unified whole in order for us to be self-conscious, but this would not be a spatio-temporal whole. Given that space and time are our forms of intuition, however, our understanding must still cooperate with sensibility to construct a spatio-temporal whole of experience because, once again, “we can represent nothing as combined in the object without having previously combined it ourselves,” and “all combination [...] is an action of the understanding” (B130). So Kant distinguishes between space and time as pure forms of intuition, which belong solely to sensibility; and the formal intuitions of space and time (or space-time), which are unified by the understanding (B160–161). These formal intuitions are the spatio-temporal whole within which our understanding constructs experience in accordance with the categories.[18]

The most important implication of Kant's claim that the understanding constructs a single whole of experience to which all of our representations can be related is that, since he defines nature “regarded materially” as “the sum total of all appearances” and he has argued that the categories are objectively valid of all possible appearances, on his view it follows that our categories are the source of the fundamental laws of nature “regarded formally” (B163, 165). So Kant concludes on this basis that the understanding is the true law-giver of nature. In his words: “all appearances in nature, as far as their combination is concerned, stand under the categories, on which nature (considered merely as nature in general) depends, as the original ground of its necessary lawfulness (as nature regarded formally)” (ibid.). Or more strongly: “we ourselves bring into the appearances that order and regularity that we call nature, and moreover we would not be able to find it there if we, or the nature of our mind, had not originally put it there. [...] The understanding is thus not merely a faculty for making rules through the comparison of the appearances: it is itself the legislation for nature, i.e., without understanding there would not be any nature at all” (A125–126).

5. Morality and freedom

Having examined two central parts of Kant's positive project in theoretical philosophy from the Critique of Pure Reason, transcendental idealism and the transcendental deduction, let us now turn to his practical philosophy in the Critique of Practical Reason. Since Kant's philosophy is deeply systematic, this section begins with a preliminary look at how his theoretical and practical philosophy fit together (see also section 7).

5.1 Theoretical and practical autonomy

The fundamental idea of Kant's philosophy is human autonomy. So far we have seen this in Kant's constructivist view of experience, according to which our understanding is the source of the general laws of nature. “Autonomy” literally means giving the law to oneself, and on Kant's view our understanding provides laws that constitute the a priori framework of our experience. Our understanding does not provide the matter or content of our experience, but it does provide the basic formal structure within which we experience any matter received through our senses. Kant's central argument for this view is the transcendental deduction, according to which it is a condition of self-consciousness that our understanding constructs experience in this way. So we may call self-consciousness the highest principle of Kant's theoretical philosophy, since it is (at least) the basis for all of our a priori knowledge about the structure of nature.

Kant's moral philosophy is also based on the idea of autonomy. He holds that there is a single fundamental principle of morality, on which all specific moral duties are based. He calls this moral law (as it is manifested to us) the categorical imperative (see 5.4). The moral law is a product of reason, for Kant, while the basic laws of nature are products of our understanding. There are important differences between the senses in which we are autonomous in constructing our experience and in morality. For example, Kant regards understanding and reason as different cognitive faculties, although he sometimes uses “reason” in a wide sense to cover both.[19] The categories and therefore the laws of nature are dependent on our specifically human forms of intuition, while reason is not. The moral law does not depend on any qualities that are peculiar to human nature but only on the nature of reason as such, although its manifestation to us as a categorical imperative (as a law of duty) reflects the fact that the human will is not necessarily determined by pure reason but is also influenced by other incentives rooted in our needs and inclinations; and our specific duties deriving from the categorical imperative do reflect human nature and the contingencies of human life. Despite these differences, however, Kant holds that we give the moral law to ourselves, just as we also give the general laws of nature to ourselves, though in a different sense. Moreover, we each necessarily give the same moral law to ourselves, just as we each construct our experience in accordance with the same categories. To summarize:

  • Theoretical philosophy is about how the world is (A633/B661). Its highest principle is self-consciousness, on which our knowledge of the basic laws of nature is based. Given sensory data, our understanding constructs experience according to these a priori laws.
  • Practical philosophy is about how the world ought to be (ibid., A800–801/B828–829). Its highest principle is the moral law, from which we derive duties that command how we ought to act in specific situations. Kant also claims that reflection on our moral duties and our need for happiness leads to the thought of an ideal world, which he calls the highest good (see section 6). Given how the world is (theoretical philosophy) and how it ought to be (practical philosophy), we aim to make the world better by constructing or realizing the highest good.

So both parts of Kant's philosophy are about autonomously constructing a world, but in different senses. In theoretical philosophy, we use our categories and forms of intuition to construct a world of experience or nature. In practical philosophy, we use the moral law to construct the idea of a moral world or a realm of ends that guides our conduct (4:433), and ultimately to transform the natural world into the highest good. Finally, transcendental idealism is the framework within which these two parts of Kant's philosophy fit together (20:311). Theoretical philosophy deals with appearances, to which our knowledge is strictly limited; and practical philosophy deals with things in themselves, although it does not give us knowledge about things in themselves but only provides rational justification for certain beliefs about them for practical purposes.

To understand Kant's arguments that practical philosophy justifies certain beliefs about things in themselves, it is necessary to see them in the context of his criticism of German rationalist metaphysics. The three traditional topics of Leibniz-Wolffian special metaphysics were rational psychology, rational cosmology, and rational theology, which dealt, respectively, with the human soul, the world-whole, and God. In the part of the Critique of Pure Reason called the Transcendental Dialectic, Kant argues against the Leibniz-Wolffian view that human beings are capable of a priori knowledge in each of these domains, and he claims that the errors of Leibniz-Wolffian metaphysics are due to an illusion that has its seat in the nature of human reason itself. According to Kant, human reason necessarily produces ideas of the soul, the world-whole, and God; and these ideas unavoidably produce the illusion that we have a priori knowledge about transcendent objects corresponding to them. This is an illusion, however, because in fact we are not capable of a priori knowledge about any such transcendent objects. Nevertheless, Kant attempts to show that these illusory ideas have a positive, practical use. He thus reframes Leibniz-Wolffian special metaphysics as a practical science that he calls the metaphysics of morals. On Kant's view, our ideas of the soul, the world-whole, and God provide the content of morally justified beliefs about human immortality, human freedom, and the existence of God, respectively; but they are not proper objects of speculative knowledge.[20]

5.2 Freedom

The most important belief about things in themselves that Kant thinks only practical philosophy can justify concerns human freedom. Freedom is important because, on Kant's view, moral appraisal presupposes that we are free in the sense that we have the ability to do otherwise. To see why, consider Kant's example of a man who commits a theft (5:95ff.). Kant holds that in order for this man's action to be morally wrong, it must have been within his control in the sense that it was within his power at the time not to have committed the theft. If this was not within his control at the time, then, while it may be useful to punish him in order to shape his behavior or to influence others, it nevertheless would not be correct to say that his action was morally wrong. Moral rightness and wrongness apply only to free agents who control their actions and have it in their power, at the time of their actions, either to act rightly or not. According to Kant, this is just common sense.

On these grounds, Kant rejects a type of compatibilism that he calls the “comparative concept of freedom” and associates with Leibniz (5:96–97). (Note that Kant has a specific type of compatibilism in mind, which I will refer to simply as “compatibilism,” although there may be other types of compatibilism that do not fit Kant's characterization of that view). On the compatibilist view, as Kant understands it, I am free whenever the cause of my action is within me. So I am unfree only when something external to me pushes or moves me, but I am free whenever the proximate cause of my body's movement is internal to me as an “acting being” (5:96). If we distinguish between involuntary convulsions and voluntary bodily movements, then on this view free actions are just voluntary bodily movements. Kant ridicules this view as a “wretched subterfuge” that tries to solve an ancient philosophical problem “with a little quibbling about words” (ibid.). This view, he says, assimilates human freedom to “the freedom of a turnspit,” or a projectile in flight, or the motion of a clock's hands (5:96–97). The proximate causes of these movements are internal to the turnspit, the projectile, and the clock at the time of the movement. This cannot be sufficient for moral responsibility.

Why not? The reason, Kant says, is ultimately that the causes of these movements occur in time. Return to the theft example. A compatibilist would say that the thief's action is free because its proximate cause is inside him, and because the theft was not an involuntary convulsion but a voluntary action. The thief decided to commit the theft, and his action flowed from this decision. According to Kant, however, if the thief's decision is a natural phenomenon that occurs in time, then it must be the effect of some cause that occurred in a previous time. This is an essential part of Kant's Newtonian worldview and is grounded in the a priori laws (specifically, the category of cause and effect) in accordance with which our understanding constructs experience: every event has a cause that begins in an earlier time. If that cause too was an event occurring in time, then it must also have a cause beginning in a still earlier time, etc. All natural events occur in time and are thoroughly determined by causal chains that stretch backwards into the distant past. So there is no room for freedom in nature, which is deterministic in a strong sense.

The root of the problem, for Kant, is time. Again, if the thief's choice to commit the theft is a natural event in time, then it is the effect of a causal chain extending into the distant past. But the past is out of his control now, in the present. Once the past is past, he can't change it. On Kant's view, that is why his actions would not be in his control in the present if they are determined by events in the past. Even if he could control those past events in the past, he cannot control them now. But in fact past events were not in his control in the past either if they too were determined by events in the more distant past, because eventually the causal antecedents of his action stretch back before his birth, and obviously events that occurred before his birth were not in his control. So if the thief's choice to commit the theft is a natural event in time, then it is not now and never was in his control, and he could not have done otherwise than to commit the theft. In that case, it would be a mistake to hold him morally responsible for it.

Compatibilism, as Kant understands it, therefore locates the issue in the wrong place. Even if the cause of my action is internal to me, if it is in the past — for example, if my action today is determined by a decision I made yesterday, or from the character I developed in childhood — then it is not within my control now. The real issue is not whether the cause of my action is internal or external to me, but whether it is in my control now. For Kant, however, the cause of my action can be within my control now only if it is not in time. This is why Kant thinks that transcendental idealism is the only way to make sense of the kind of freedom that morality requires. For transcendental idealism allows that the cause of my action may be a thing in itself outside of time: namely, my noumenal self, which is free because it is not part of nature. No matter what kind of character I have developed or what external influences act on me, on Kant's view all of my intentional, voluntary actions are immediate effects of my noumenal self, which is causally undetermined (5:97–98). My noumenal self is an uncaused cause outside of time, which therefore is not subject to the deterministic laws of nature in accordance with which our understanding constructs experience.

Many puzzles arise on this picture that Kant does not resolve. For example, if my understanding constructs all appearances in my experience of nature, not only appearances of my own actions, then why am I responsible only for my own actions but not for everything that happens in the natural world? Moreover, if I am not alone in the world but there are many noumenal selves acting freely and incorporating their free actions into the experience they construct, then how do multiple transcendentally free agents interact? How do you integrate my free actions into the experience that your understanding constructs?[21] In spite of these unsolved puzzles, Kant holds that we can make sense of moral appraisal and responsibility only by thinking about human freedom in this way, because it is the only way to prevent natural necessity from undermining both.

Finally, since Kant invokes transcendental idealism to make sense of freedom, interpreting his thinking about freedom leads us back to disputes between the two-objects and two-aspects interpretations of transcendental idealism. On the face of it, the two-objects interpretation seems to make better sense of Kant's view of transcendental freedom than the two-aspects interpretation. If morality requires that I am transcendentally free, then it seems that my true self, and not just an aspect of my self, must be outside of time, according to Kant's argument. But applying the two-objects interpretation to freedom raises problems of its own, since it involves making a distinction between noumenal and phenomenal selves that does not arise on the two-aspects view. If only my noumenal self is free, and freedom is required for moral responsibility, then my phenomenal self is not morally responsible. But how are my noumenal and phenomenal selves related, and why is punishment inflicted on phenomenal selves? It is unclear whether and to what extent appealing to Kant's theory of freedom can help to settle disputes about the proper interpretation of transcendental idealism, since there are serious questions about the coherence of Kant's theory on either interpretation.

5.3 The fact of reason

Can we know that we are free in this transcendental sense? Kant's response is tricky. On the one hand, he distinguishes between theoretical knowledge and morally justified belief (A820–831/B848–859). We do not have theoretical knowledge that we are free or about anything beyond the limits of possible experience, but we are morally justified in believing that we are free in this sense. On the other hand, Kant also uses stronger language than this when discussing freedom. For example, he says that “among all the ideas of speculative reason freedom is the only one the possibility of which we know a priori, though without having any insight into it, because it is the condition of the moral law, which we do know.” In a footnote to this passage, Kant explains that we know freedom a priori because “were there no freedom, the moral law would not be encountered at all in ourselves,” and on Kant's view everyone does encounter the moral law a priori (5:4). For this reason, Kant claims that the moral law “proves” the objective, “though only practical, undoubted reality” of freedom (5:48–49). So Kant wants to say that we do have knowledge of the reality of freedom, but that this is practical knowledge of a practical reality, or cognition “only for practical purposes,” by which he means to distinguish it from theoretical knowledge based on experience or reflection on the conditions of experience (5:133). Our practical knowledge of freedom is based instead on the moral law. The difference between Kant's stronger and weaker language seems mainly to be that his stronger language emphasizes that our belief or practical knowledge about freedom is unshakeable and that it in turn provides support for other morally grounded beliefs in God and the immortality of the soul.

Kant calls our consciousness of the moral law, our awareness that the moral law binds us or has authority over us, the “fact of reason” (5:31–32, 42–43, 47, 55). So, on his view, the fact of reason is the practical basis for our belief or practical knowledge that we are free. Kant insists that this moral consciousness is “undeniable,” “a priori,” and “unavoidable” (5:32, 47, 55). Every human being has a conscience, a common sense grasp of morality, and a firm conviction that he or she is morally accountable. We may have different beliefs about the source of morality's authority — God, social convention, human reason. We may arrive at different conclusions about what morality requires in specific situations. And we may violate our own sense of duty. But we all have a conscience, and an unshakeable belief that morality applies to us. According to Kant, this belief cannot and does not need to be justified or “proved by any deduction” (5:47). It is just a ground-level fact about human beings that we hold ourselves morally accountable. But Kant is making a normative claim here as well: it is also a fact, which cannot and does not need to be justified, that we are morally accountable, that morality does have authority over us. Kant holds that philosophy should be in the business of defending this common sense moral belief, and that in any case it could never prove or disprove it (4:459).

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