Recent Modern Philosophy: Immanuel Kant

By Dr. Garth Kemerling / 11.12.2011
Professor of Philosophy
Capella University

Philosophy Pages

Life and Works

Immanuel Kant was born in the East Prussian city of Königsberg, studied at its university, and worked there as a tutor and professor for more than forty years, never travelling more than fifty miles from home. Although his outward life was one of legendary calm and regularity, Kant’s intellectual work easily justified his own claim to have effected a Copernican revolution in philosophy. Beginning with his Inaugural Dissertation (1770) on the difference between right- and left-handed spatial orientations, Kant patiently worked out the most comprehensive and influential philosophical programme of the modern era. His central thesis—that the possibility of human knowledge presupposes the active participation of the human mind—is deceptively simple, but the details of its application are notoriously complex.

The monumental Kritik der reinen Vernunft (Critique of Pure Reason) (1781, 1787) fully spells out the conditions for mathematical, scientific, and metaphysical knowledge in its “Transcendental Aesthetic,” “Transcendental Analytic,” and “Transcendental Dialectic,” but Kant found it helpful to offer a less technical exposition of the same themes in theProlegomena zu einer jeden künftigen Metaphysik die als Wissenschaft wird auftreten können (Prolegomena to any Future Metaphysic) (1783). Carefully distinguishing judgments as analytic or synthetic and as a priori or a posteriori, Kant held that the most interesting and useful varieties of human knowledge rely upon synthetic a priori judgments, which are, in turn, possible only when the mind determines the conditions of its own experience. Thus, it is we who impose the forms of space and time upon all possible sensation in mathematics, and it is we who render all experience coherent as scientific knowledge governed by traditional notions of substance and causality by applying the pure concepts of the understanding to all possible experience.   But regulative principles of this sort hold only for the world as we know it, and since metaphysical propositions seek a truthbeyond all experience, they cannot be established within the bounds of reason.

Significant applications of these principles are expressed in Metaphysische Anfangsgründe der Naturwissenschaft (Metaphysical Foundations of the Science of Nature) (1786) and Beantwortung der Frage: Ist es eine Erfahrung, daß wir denken? (On Comprehension and Transcendental Consciousness) (1788-1791).

Kant’s moral philosophy is developed in the Grundlegung zur Metaphysik der Sitten (Grounding for the Metaphysics of Morals) (1785). From his analysis of the operation of the human will, Kant derived the necessity of a perfectly universalizable moral law, expressed in acategorical imperative that must be regarded as binding upon every agent. In the Third Section of the Grounding and in the Kritik der practischen Vernunft(Critique of Practical Reason) (1788), Kant grounded this conception of moral autonomy upon our postulation of god, freedom, and immortality.

In later life, Kant drew art and science together under the concept of purpose in the Kritik der Urteilskraft (Critique of Judgment) (1790), considered the consequences of transcendental criticism for theology in Die Religion innerhalb die Grenzen der blossen Vernunft (Religion within the Limits of Reason Alone) (1793), stated the fundamental principles for civil discourse in Beantwortung der Frage: Was ist Aufklärung? (“What is Enlightenment?” (1784), and made an eloquent plea for international cooperation in Zum ewigen Frieden (Perpetual Peace) (1795).

The Critical Philosophy

Next we turn to the philosophy of Immanuel Kant, a watershed figure who forever altered the course of philosophical thinking in the Western tradition. Long after his thorough indoctrination into the quasi-scholastic German appreciation of the metaphysical systems of Leibniz and Wolff, Kant said, it was a careful reading of David Hume that “interrupted my dogmatic slumbers and gave my investigations in the field of speculative philosophy a quite new direction.” Having appreciated the full force of such skeptical arguments, Kant supposed that the only adequate response would be a “Copernican Revolution” in philosophy, a recognition that the appearance of the external world depends in some measure upon the position and movement of its observers. This central idea became the basis for his life-long project of developing a critical philosophy that could withstand them.

Kant’s aim was to move beyond the traditional dichotomy between rationalism and empiricism. The rationalists had tried to show that we can understand the world by careful use of reason; this guarantees the indubitability of our knowledge but leaves serious questions about its practical content. The empiricists, on the other hand, had argued that all of our knowledge must be firmly grounded in experience; practical content is thus secured, but it turns out that we can be certain of very little. Both approaches have failed, Kant supposed, because both are premised on the same mistaken assumption.

Progress in philosophy, according to Kant, requires that we frame the epistemological problem in an entirely different way. The crucial question is not how we can bring ourselves to understand the world, but how the world comes to be understood by us. Instead of trying, by reason or experience, to make our concepts match the nature of objects, Kant held, we must allow the structure of our concepts shape our experience of objects. This is the purpose of Kant’s Critique of Pure Reason (1781, 1787): to show how reason determines the conditions under which experience and knowledge are possible.

Varieties of Judgment

In the Prolegomena to any Future Metaphysic (1783) Kant presented the central themes of the first Critique in a somewhat different manner, starting from instances in which we do appear to have achieved knowledge and asking under what conditions each case becomes possible. So he began by carefully drawing a pair of crucial distinctions among the judgments we do actually make.

The first distinction separates a priori from a posteriori judgments by reference to the origin of our knowledge of them. A priori judgments are based upon reason alone, independently of all sensory experience, and therefore apply with strict universality. A posteriori judgments, on the other hand, must be grounded upon experience and are consequently limited and uncertain in their application to specific cases. Thus, this distinction also marks the difference traditionally noted in logic between necessary and contingent truths.

But Kant also made a less familiar distinction between analytic and syntheticjudgments, according to the information conveyed as their content. Analyticjudgments are those whose predicates are wholly contained in their subjects; since they add nothing to our concept of the subject, such judgments are purely explicative and can be deduced from the principle of non-contradiction.Synthetic judgments, on the other hand, are those whose predicates are wholly distinct from their subjects, to which they must be shown to relate because of some real connection external to the concepts themselves. Hence, synthetic judgments are genuinely informative but require justification by reference to some outside principle.

Kant supposed that previous philosophers had failed to differentiate properly between these two distinctions. Both Leibniz and Hume had made just one distinction, between matters of fact based on sensory experience and the uninformative truths of pure reason. In fact, Kant held, the two distinctions are not entirely coextensive; we need at least to consider all four of their logically possible combinations:

Analytic a posteriori judgments cannot arise, since there is never any need to appeal to experience in support of a purely explicative assertion.

Synthetic a posteriori judgments are the relatively uncontroversial matters of fact we come to know by means of our sensory experience (though Wolff had tried to derive even these from the principle of contradiction).

Analytic a priori judgments, everyone agrees, include all merely logical truths and straightforward matters of definition; they are necessarily true.

Synthetic a priori judgments are the crucial case, since only they could provide new information that is necessarily true. But neither Leibniz nor Hume considered the possibility of any such case.

Unlike his predecessors, Kant maintained that synthetic a priori judgments not only are possible but actually provide the basis for significant portions of human knowledge. In fact, he supposed (pace Hume) that arithmetic and geometry comprise such judgments and that natural science depends on them for its power to explain and predict events. What is more, metaphysics—if it turns out to be possible at all—must rest upon synthetic a priori judgments, since anything else would be either uninformative or unjustifiable. But how are synthetic a priori judgments possible at all? This is the central question Kant sought to answer.


Consider, for example, our knowledge that two plus three is equal to five and that the interior angles of any triangle add up to a straight line. These (and similar) truths of mathematics are synthetic judgments, Kant held, since they contribute significantly to our knowledge of the world; the sum of the interior angles is not contained in the concept of a triangle. Yet, clearly, such truths are known a priori, since they apply with strict and universal necessity to all of the objects of our experience, without having been derived from that experience itself. In these instances, Kant supposed, no one will ask whether or not we have synthetic a priori knowledge; plainly, we do. The question is, how do we come to have such knowledge? If experience does not supply the required connection between the concepts involved, what does?

Kant’s answer is that we do it ourselves. Conformity with the truths of mathematics is a precondition that we impose upon every possible object of our experience. Just as Descartes had noted in the Fifth Meditation, the essence of bodies is manifested to us in Euclidean solid geometry, which determines a priori the structure of the spatial world we experience. In order to be perceived by us, any object must be regarded as being uniquely located in space and time, so it is the spatio-temporal framework itself that provides the missing connection between the concept of the triangle and that of the sum of its angles. Space and time, Kant argued in the “Transcendental Aesthetic” of the first Critique, are the “pure forms of sensible intuition” under which we perceive what we do.

Understanding mathematics in this way makes it possible to rise above an old controversy between rationalists and empiricists regarding the very nature of space and time. Leibniz had maintained that space and time are not intrinsic features of the world itself, but merely a product of our minds. Newton, on the other hand, had insisted that space and time are absolute, not merely a set of spatial and temporal relations. Kant now declares that both of them were correct! Space and time are absolute, and they do derive from our minds. As synthetic a priori judgments, the truths of mathematics are both informative and necessary.

This is our first instance of a transcendental argument, Kant’s method of reasoning from the fact that we have knowledge of a particular sort to the conclusion that all of the logical presuppositions of such knowledge must be satisfied. We will see additional examples in later lessons, and can defer our assessment of them until then. But notice that there is a price to be paid for the certainty we achieve in this manner. Since mathematics derives from our own sensible intuition, we can be absolutely sure that it must apply to everything we perceive, but for the same reason we can have no assurance that it has anything to do with the way things are apart from our perception of them. Next time, we’ll look at Kant’s very similar treatment of the synthetic a priori principles upon which our knowledge of natural science depends.

Preconditions for Natural Science

In natural science no less than in mathematics, Kant held, synthetic a priorijudgments provide the necessary foundations for human knowledge. The most general laws of nature, like the truths of mathematics, cannot be justified by experience, yet must apply to it universally. In this case, the negative portion of Hume’s analysis—his demonstration that matters of fact rest upon an unjustifiable belief that there is a necessary connection between causes and their effects—was entirely correct. But of course Kant’s more constructive approach is to offer a transcendental argument from the fact that we do have knowledge of the natural world to the truth of synthetic a priori propositions about the structure of our experience of it.

As we saw last time, applying the concepts of space and time as forms of sensible intuition is necessary condition for any perception. But the possibility of scientific knowledge requires that our experience of the world be not only perceivable but thinkable as well, and Kant held that the general intelligibility of experience entails the satisfaction of two further conditions:

First, it must be possible in principle to arrange and organize the chaos of our many individual sensory images by tracing the connections that hold among them. This Kant called the synthetic unity of the sensory manifold.

Second, it must be possible in principle for a single subject to perform this organization by discovering the connections among perceived images. This is satisfied by what Kant called the transcendental unity of apperception.
Experiential knowledge is thinkable only if there is some regularity in what is known and there is some knower in whom that regularity can be represented. Since we do actually have knowledge of the world as we experience it, Kant held, both of these conditions must in fact obtain.

Deduction of the Categories

Since (as Hume had noted) individual images are perfectly separable as they occur within the sensory manifold, connections between them can be drawn only by the knowing subject, in which the principles of connection are to be found. As in mathematics, so in science the synthetic a priori judgments must derive from the structure of the understanding itself.

Consider, then, the sorts of judgments distinguished by logicians (in Kant’s day): each of them has some quantity (applying to all things, some, or only one); some quality (affirmative, negative, or complementary); some relation (absolute, conditional, or alternative); and some modality (problematic, assertoric, or apodeictic). Kant supposed that any intelligible thought can be expressed in judgments of these sorts. But then it follows that any thinkable experience must be understood in these ways, and we are justified in projecting this entire way of thinking outside ourselves, as the inevitable structure of any possible experience.

The result of this “Transcendental Logic” is the schematized table of categories, Kant’s summary of the central concepts we employ in thinking about the world, each of which is discussed in a separate section of the Critique:

Quantity                                           Quality

Unity                                                   Reality
Plurality                                               Negation
Totality                                                 Limitation
Axioms of Intuition                            Anticipations of Perception

Relation                                                          Modality

Substance                                                         Possibility
Cause                                                                 Existence
Community                                                            Necessity
Analogies of Experience                                 Postulates of Empirical Thought

Our most fundamental convictions about the natural world derive from these concepts, according to Kant. The most general principles of natural science are not empirical generalizations from what we have experienced, but synthetic a priori judgments about what we could experience, in which these concepts provide the crucial connectives.

Experience and Reality

Analogies of Experience

So Kant maintained that we are justified in applying the concepts of the understanding to the world as we know it by making a priori determinations of the nature of any possible experience. In order to see how this works in greater detail, let’s concentrate on the concepts of relation, which govern how we understand the world in time. As applied in the Analogies of Experience, each concept of relation establishes one of the preconditions of experience under one of the modes of time: duration, succession, and simultaneity.

1. Substance: The experience of any change requires not only the perception of the altered qualities that constitute the change but also the concept of an underlying substance which persists through this alteration. (E.g., in order to know by experience that the classroom wall has changed in color from blue to yellow, I must not only perceive the different colors—blue then, yellow now—but also suppose that the wall itself has endured from then until now.) Thus, Kant supposed that the philosophical concept of substance (reflected in the scientific assumption of an external world of material objects) is an a prioricondition for our experience.

2. Cause: What is more, the experience of events requires not only awareness of their intrinsic features but also that they be regarded as occurring one after another, in an invariable regularity determined by the concept of causality. (E.g., in order to experience the flowering of this azalea as an event, I must not only perceive the blossoms as they now appear but must also regard them as merely the present consequence of a succession of prior organic developments.) Thus, Kant responded to Hume’s skepticism by maintaining that the concept of cause is one of the synthetic conditions we determine for ourselves prior to all experience.

3. Community: Finally, the experience of a world of coexisting things requires not only the experiences of each individually but also the presumption of their mutual interaction. (E.g., in order believe that the Sun, Earth, and Moon coexist in a common solar system, I must not only make some estimate of the mass of each but must also take into account the reciprocity of the gravitational forces between them.) Thus, on Kant’s view, the notion of the natural world as a closed system of reciprocal forces is another a priori condition for the intelligibility of experience.

Notice again that these features of nature are not generalized from anything we have already experienced; they are regulative principles that we impose in advance on everything we can experience. We are justified in doing so, Kant believed, because only the pure concepts of the understanding can provide the required connections to establish synthetic a priori judgments. Unless these concepts are systematically applied to the sensory manifold, the unity of apperception cannot be achieved, and no experience can be made intelligible.

Phenomena and Noumena

Having seen Kant’s transcendental deduction of the categories as pure concepts of the understanding applicable a priori to every possible experience, we might naturally wish to ask the further question whether these regulative principles are really true. Are there substances? Does every event have a cause? Do all things interact? Given that we must suppose them in order to have any experience, do they obtain in the world itself? To these further questions, Kant firmly refused to offer any answer.

According to Kant, it is vital always to distinguish between the distinct realms of phenomena and noumena. Phenomena are the appearances, which constitute the our experience; noumena are the (presumed) things themselves, which constitute reality. All of our synthetic a priori judgments apply only to the phenomenal realm, not the noumenal. (It is only at this level, with respect to what we can experience, that we are justified in imposing the structure of our concepts onto the objects of our knowledge.) Since the thing in itself (Ding an sich) would by definition be entirely independent of our experience of it, we are utterly ignorant of the noumenal realm.

Thus, on Kant’s view, the most fundamental laws of nature, like the truths of mathematics, are knowable precisely because they make no effort to describe the world as it really is but rather prescribe the structure of the world as we experience it. By applying the pure forms of sensible intuition and the pure concepts of the understanding, we achieve a systematic view of the phenomenal realm but learn nothing of the noumenal realm. Math and science are certainly true of the phenomena; only metaphysics claims to instruct us about the noumena.

The Aim of Metaphysics

Although our knowledge of mathematics and natural science yield easily to a Kantian analysis, the synthetic a priori judgments of metaphysics are much more difficult to explain. Here the forms of intuition and concepts of understanding are useless, since they find application only in the realm of our experience, while metaphysics seeks to transcend experience completely, in order to discover the nature of reality itself as comprehended under pure reason.

Metaphysical speculation properly begins with the same method as the “Aesthetic” and “Analytic,” Kant supposed, but it invariably ends up in a “Dialectic.” The transcendental arguments we employ in metaphysics need not restrict their determination to the phenomenal realm alone, since their aim is genuine knowledge of the noumena. Synthetic a priori judgments in metaphysics must be grounded upon truly transcendental ideas, which are regarded as applicable to things in themselves independently of our experience of them.

Transcendental Ideas

Kant’s exposition of the transcendental ideas begins once again from the logical distinction among categorical, hypothetical, and disjunctive syllogisms. From this distinction, as we have seen, the understanding derives the concepts of substance, cause, and community, which provide the basis for rules that obtain as natural laws within our experience. Now, from the same distinction, the reason must carry things further in order derive the transcendental ideas of the complete subject, the complete series of conditions, and the complete complex of what is possible. Thus, the “completion” of metaphysical reasoning requires transcendental ideas of three sorts, but Kant argued that each leads to its characteristic irresolvable difficulty.

The Psychological Idea is the concept of the soul as a permanent substance which lives forever. It is entirely natural to reason (as in Descartes’s cogito) from knowledge that “I think” to my real existence as one and the same thinking thing through all time, but Kant held that our efforts to reach such conclusions are “Paralogisms,” with only illusory validity. It is true that thought presupposes the unity of apperception and that every change presupposes an underlying substance, but these rules apply only to the phenomena we experience. Since substantial unity and immortality are supposed to be noumenal features of the soul as a thing in itself, Kant held, legitimate a priorijudgments can never prove them, and the effort to transcend in this case fails.

The Cosmological Idea is the concept of a complete determination of the nature of the world as it must be constituted in itself. In this case, Kant held, the difficulty is not that we can conclude too little but rather that we can prove too much. From the structure of our experience of the world, it is easy to deduce contradictory particular claims about reality: finitude vs. infinity; simplicity vs. complexity; freedom vs. determinism; necessity vs. contingency. These “Antinomies” of Pure Reason can be avoided only when we recognize that one or both of the contradictory proofs in each antinomy holds only for the phenomenal realm. Once again, it is the effort to achieve transcendental knowledge of noumena that necessarily fails.

The Theological Idea is the concept of an absolutely perfect and most real being (or god). Again it is natural to move from our recognition of dependence within the phenomenal realm to the notion of a perfectly independent noumenal being, the “Transcendental Ideal.” But traditional attempts to prove that god really exists, founded as they are on what we experience, cannot establish the reality of a being necessarily beyond all experience.

The general point of the Transcendental Dialectic should by now be clear: metaphysical speculation about the ultimate nature of reality invariably fails. The synthetic a priori judgments which properly serve as regulative principles governing our experience can never be shown to have any force as constitutive of the real nature of the world. Pure reason inevitably reaches for what it cannot grasp.

The Limits of Reason

Now that we’ve seen Kant’s answers to all three parts of the Prolegomena’s “Main Transcendental Question” and have traced their sources in the Critique of Pure Reason, we are in a position to appreciate his careful delineation of what is possible in metaphysical thought and what is not.

What most clearly is not possible is any legitimate synthetic a priorijudgment about things in themselves. The only thing that justifies the application of regulative principles in mathematics and natural science is their limitation to phenomena. Both sensible intuition and the understanding deal with the conditions under which experience is possible. But the whole point of speculative metaphysics is to transcend experience entirely in order to achieve knowledge of the noumenal realm. Here, only the faculty of reason is relevant, but its most crucial speculative conclusions, its deepest convictions about the self, the world, and god, are all drawn illegitimately.

What is possible—indeed, according to Kant what we are bound by our very nature as rational beings to do—is to think of the noumenal realm as if the speculative principles were true (whether or not they are). By the nature of reason itself, we are required to suppose our own existence as substantial beings, the possibility of our free action in a world of causal regularity, and the existence of god. The absence of any formal justification for these notions makes it impossible for us to claim that we know them to be true, but it can in no way diminish the depth fo our belief that they are.

According to Kant, then, the rational human faculties lead us to the very boundaries of what can be known, by clarifying the conditions under which experience of the world as we know it is possible. But beyond those boundaries our faculties are useless. The shape of the boundary itself, as evidenced in the Paralogisms and Antinomies, naturally impels us to postulate that the unknown does indeed have certain features, but these further speculations are inherently unjustifiable.

The only legitimate, “scientific” metaphysics that the future may hold, Kant therefore held, would be a thoroughly critical, non-speculative examination of the bounds of pure reason, a careful description of what we can know accompanied by a clear recognition that our transcendental concepts (however useful they may seem) are entirely unreliable as guides to the nature of reality. It is this task, of course, that Kant himself had pursued in the First Critique.

The Moral Order

Having mastered epistemology and metaphysics, Kant believed that a rigorous application of the same methods of reasoning would yield an equal success in dealing with the problems of moral philosophy. Thus, in the Kritik der practischen Vernunft (Critique of Practical Reason) (1788), he proposed a “Table of the Categories of Freedom in Relation to the Concepts of Good and Evil,” using the familiar logical distinctions as the basis for a catalog of synthetic a priori judgments that have bearing on the evaluation of human action, and declared that only two things inspire genuine awe: “der bestirnte Himmel über mir und das moralische Gesetz in mir” (“the starry sky above and the moral law within”). Kant used ordinary moral notions as the foundation ffor a derivation of this moral law in his Grundlegung zur Metaphysik der Sitten(Grounding for the Metaphysics of Morals) (1785).

From Good Will to Universal Law

We begin with the concept of that which can be conceived to be good without qualification, a good will. Other good features of human nature and the benefits of a good life, Kant pointed out, have value only under appropriate conditions, since they may be used either for good or for evil. But a good will is intrinsically good; its value is wholly self-contained and utterly independent of its external relations. Since our practical reason is better suited to the development and guidance of a good will than to the achievement of happiness, it follows that the value of a good will does not depend even on the results it manages to produce as the consequences of human action.

Kant’s moral theory is, therefore, deontological: actions are morally right in virtue of their motives, which must derive more from duty than from inclination. The clearest examples of morally right action are precisely those in which an individual agent’s determination to act in accordance with duty overcomes her evident self-interest and obvious desire to do otherwise. But in such a case, Kant argues, the moral value of the action can only reside in a formal principle or “maxim,” the general commitment to act in this way because it is one’s duty. So he concludes that “Duty is the necessity to act out of reverence for the law.”

According to Kant, then, the ultimate principle of morality must be a moral law conceived so abstractly that it is capable of guiding us to the right action in application to every possible set of circumstances. So the only relevant feature of the moral law is its generality, the fact that it has the formal property ofuniversalizability, by virtue of which it can be applied at all times to every moral agent. From this chain of reasoning about our ordinary moral concepts, Kant derived as a preliminary statement of moral obligation the notion that right actions are those that practical reason would will as universal law.

Imperatives for Action

More accurate comprehension of morality, of course, requires the introduction of a more precise philosophical vocabulary. Although everything naturally acts in accordance with law, Kant supposed, only rational beings do so consciously, in obedience to the objective principles determined by practical reason. Of course, human agents also have subjective impulses—desires and inclinations that may contradict the dictates of reason. So we experience the claim of reason as an obligation, a command that we act in a particular way, or an imperative. Such imperatives may occur in either of two distinct forms,hypothetical or categorical.

A hypothetical imperative conditionally demands performance of an action for the sake of some other end or purpose; it has the form “Do A in order to achieve X.” The application of hypothetical imperatives to ethical decisions is mildly troublesome: in such cases it is clear that we are morally obliged to perform the action A only if we are sure both that X is a legitimate goal andthat doing A will in fact produce this desirable result. For a perfectly rational being, all of this would be analytic, but given the general limitations of human knowledge, the joint conditions may rarely be satisfied.

A categorical imperative, on the other hand, unconditionally demands performance of an action for its own sake; it has the form “Do A.” An absolute moral demand of this sort gives rise to familiar difficulties: since it expresses moral obligation with the perfect necessity that would directly bind any will uncluttered by subjective inclinations, the categorical imperative must be known a priori; yet it cannot be an analytic judgment, since its content is not contained in the concept of a rational agent as such. The supreme principle of morality must be a synthetic a priori proposition. Leaving its justification for the third section of the Grounding (and the Second Critique), Kant proceeded to a discussion of the content and application of the categorical impetative.

The Categorical Imperative

Constrained only by the principle of universalizability, the practical reason of any rational being understands the categorical imperative to be: “Act only according to that maxim whereby you can at the same time will that it should become a universal law.” That is, each individual agent regards itself as determining, by its decision to act in a certain way, that everyone (including itself) will always act according to the same general rule in the future. This expression of the moral law, Kant maintained, provides a concrete, practical method for evaluating particular human actions of several distinct varieties.

Consider, for example, the case (#2 in the text) of someone who contemplates relieving a financial crisis by borrowing money from someone else, promising to repay it in the future while in fact having no intention of doing so. (Notice that this is not the case of finding yourself incapable of keeping a promise originally made in good faith, which would require a different analysis.) The maxim of this action would be that it is permissible to borrow money under false pretenses if you really need it. But as Kant pointed out, making this maxim into a universal law would be clearly self-defeating. The entire practice of lending money on promise presupposes at least the honest intention to repay; if this condition were universally ignored, the (universally) false promises would never be effective as methods of borrowing. Since the universalized maxim is contradictory in and of itself, no one could will it to be law, and Kant concluded that we have a perfect duty (to which there can never be any exceptions whatsoever) not to act in this manner.

On the other hand, consider the less obvious case (#4 in the text) of someone who lives comfortably but contemplates refusing any assistance to people who are struggling under great hardships. The maxim here would be that it is permissible never to help those who are less well-off than ourselves. Although Kant conceded that no direct contradiction would result from the universalization of such a rule of conduct, he argued that no one could consistently will that it become the universal law, since even the most fortunate among us rightly allow for the possibility that we may at some future time find ourselves in need of the benevolence of others. Here we have only an imperfect duty not act so selfishly, since particular instances may require exceptions to the rule when it conflicts either with another imperfect duty (e.g., when I don’t have enough money to help everyone in need) or a perfect duty (e.g., if the only way to get more money would be under a false promise).

Kant also supposed that moral obligations arise even when other people are not involved. Since it would be contradictory to universalize the maxim of taking one’s own life if it promises more misery than satisfaction (#1), he argued, we have a perfect duty to ourselves not to commit suicide. And since no one would will a universalized maxim of neglecting to develop the discipline required for fulfilling one’s natural abilities (#3), we have an imperfect duty to ourselves not to waste our talents.

These are only examples of what a detailed application of the moral law would entail, but they illustrate the general drift of Kant’s moral theory. In cases of each of the four sorts, he held that there is a contradiction—either in the maxim itself or in the will—involved in any attempt to make the rule under which we act into a universal law. The essence of immorality, then, is to make an exception of myself by acting on maxims that I cannot willfully universalize. It is always wrong to act in one way while wishing that everyone else would act otherwise. (The perfect world for a thief would be one in which everyone else always respected private property.) Thus, the purely formal expression of the categorical imperative is shown to yield significant practical application to moral decisions.

Alternative Formulae for the Categorical Imperative

Although he held that there is only one categorical imperative of morality, Kant found it helpful to express it in several ways. Some of the alternative statements can be regarded as minor variations on his major themes, but two differ from the “formula of universal law” sufficiently to warrant a brief independent discussion.

Kant offered the “formula of the end in itself” as: “Act in such a way that you treat humanity, whether in your own person or in the person of another, always at the same time as an end and never simply as a means.” This places more emphasis on the unique value of human life as deserving of our ultimate moral respect and thus proposes a more personal view of morality. In application to particular cases, of course, it yields the same results: violating a perfect duty by making a false promise (or killing myself) would be to treat another person (or myself) merely as a means for getting money (or avoiding pain), and violating an imperfect duty by refusing to offer benevolence (or neglecting my talents) would be a failure to treat another person (or myself) as an end in itself. Thus, the Kantian imperative agrees with the Christian expression of “The Golden Rule” by demanding that we derive from our own self-interest a generalized concern for all human beings.

Drawing everything together, Kant arrived at the “formula of autonomy,” under which the decision to act according to a maxim is actually regarded as having made it a universal law. Here the concern with human dignity is combined with the principle of universalizability to produce a conception of the moral law as self-legislated by each for all. As Kant puts it,

A rational being belongs to the kingdom of ends as a member when he legislates in it universal laws while also being himself subject to these laws. He belongs to it as sovereign, when as legislator he is himslf subject to the will of no other.

A rational being must always regard himself as legislator in a kingdom of ends rendered possible by freedom of the will, whether as member or as sovereign.

In this final formulation, the similarity of Kant’s moral theory with his epistemology should be clear. Just as the understanding in each of us determines the regulative principles of natural science that all must share, so the practical reason in each of us determines the universal maxims of morality that all must obey.

Autonomy of the Will

In fact, this final formula for the categorical imperative brings us back to the original concept of the will itself as that which is good without qualification. At this point in the argument, Kant can provide a more technical statement of its intrinsic moral value by distinguishing between autonomy and heteronomy of the will.

A heteronomous will is one in obedience to rules of action that have been legislated externally to it. Such a will is always submitting itself to some other end, and the principles of its action will invariably be hypothetical imperatives urging that it act in such a way as to receive pleasure, appease the moral sense, or seek personal perfection. In any case, the moral obligations it proposes cannot be regarded as completely binding upon any agent, since their maxim of action comes from outside it.

An autonomous will, on the other hand, is entirely self-legislating: The moral obligations by which it is perfectly bound are those which it has imposed upon itself while simultaneously regarding them as binding upon everyone else by virtue of their common possession of the same rational faculties. All genuinely moral action, Kant supposed, flows from the freely chosen dictates of an autonomous will. So even the possibility of morality presupposes that human agents have free will, and the final section of the Grounding is devoted to Kant’s effort to prove that they do.

Human Freedom

As we might expect, Kant offered as proof of human freedom a transcendental argument from the fact of moral agency to the truth of its presupposed condition of free will. This may seem to be perfectly analogous to the use of similar arguments for synthetic a priori judgments in the First Critique, but the procedure is more viciously circular here. Having demonstrated the supreme principle of morality by reference to autonomy, Kant can hardly now claim to ground free will upon the supposed fact of morality. That would be to exceed the bounds of reason by employing an epistemological argument for metaphysical purposes.

Here’s another way of looking at it: Each case of moral action may be said to embody its own unique instance of the antinomy between freedom and causal determination. For in order to do the right thing, it must at least be possible for my action to have some real effect in the world, yet I must perform it in complete independence from any external influence. Morality requires both freedom and causality in me, and of course Kant supposes that they are. I can think of myself from two standpoints: I operate within the phenomenal realm by participating fully in the causal regularities to which it is subject; but as a timeless thing in itself in the noumenal realm I must be wholly free. The trick is to think of myself in both ways at once, as sensibly determined but intelligibly free.

Kant rightly confesses at the end of the Grounding that serious contemplation of morality leads us to the very limits of human reason. Since action in accordance with the moral law requires an autonomous will, we must suppose ourselves to be free; since the correspondence of happiness with virtue cannot be left to mere coincidence, we must suppose that there is a god who guarantees it; and since the moral perfection demanded by the categorical imperative cannot be attained in this life, we must suppose ourselves to live forever. Thus god, freedom, and immortality, which we have seen to be metaphysical illusions that lie beyond the reach of pure reason, turn out to be the three great postulates of practical reason.

Although the truth about ourselves and god as noumenal beings can never be determined with perfect certainty, on Kant’s view, we can continue to function as responsible moral agents only by acting as if it obtains. Things could hardly have been otherwise: the lofty dignity of the moral law, like the ultimate nature of reality, is the sort of thing we cannot know but are bound to believe.

Morality and Peace

Kant’s interest in moral matters was not exclusively theoretical. In Die Metaphysik der Sitten (Metaphysics of Morals) (1797) he worked out the practical application of the categorical imperative in some detail, deriving a fairly comprehensive catalog of specific rules for the governance of social and personal morality. What each of us must actually will as universal, Kant supposed, is a very rigid system of narrowly prescribed conduct.

In Zum ewigen Frieden (On Perpetual Peace) (1795), Kant proposed a high-minded scheme for securing widespread political stability and security. If statesmen would listen to philosophers, he argued, we could easily achieve an international federation of independent republics, each of which reduces its standing army, declines to interfere in the internal affairs of other states, and agrees to be governed by the notion of universal hospitality.

Kant’s Third Critque

The final component of Kant’s critical philosophy found expression in his (Kritik der Urteilskraft (Critique of Judgment)1790). Where the first Critiquehad dealt with understanding in relation to reality and the second had been concerned with practical reason in relation to action, this third Critique was meant to show that there is a systematic connection between the two, a common feature underlying every use of synthetic a priori judgments, namely the concept of purpose. In the last analysis, Kant supposed, it is our compulsion to find meaning and purpose in the world that impels us to accept the tenets of transcendental idealism.

In aesthetics, for example, all of our judgments about what is beautiful or sublime derive from the determination to impose an underlying form on the sensory manifold. Like mathematics, art is concerned with the discovery or creation of unity in our experience of the spatio-temporal world. Teleological judgments in science, theology, and morality similarly depend upon our fundamental convictions, that operation of the universe has some deep purpose and that we are capable of comprehending it.

Kant’s final word here offers an explanation of our persistent desire to transcend from the phenomenal realm to the noumenal. We must impose the forms of space and time on all we perceive, we must suppose that the world we experience functions according to natural laws, we must regulate our conduct by reference to a self-legislated categorical imperative, and we must postulate the noumenal reality of ourselves, god, and free will—all because a failure to do so would be an implicit confession that the world may be meaningless, and that would be utterly intolerable for us. Thus, Kant believed, the ultimate worth of his philosophy lay in his willingness “to criticize reason in order to make room for faith.” The nineteenth-century German philosophers who followed him quickly moved to transform his modest critical philosophy into the monumental metaphysical system of absolute idealism.