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Kant's philosophy is generally designated as a system of transcendental criticism tending towards Agnosticism in theology, and favouring the view that Christianity is a non-dogmatic religion.

Immanuel Kant was born at Königsberg in East Prussia, 22 April, 1724; died there, 12 February, 1804. From his sixteenth to his twenty-first year, he studied at the university of his native city, having for his teacher Martin Knutzen, under whom he acquired a knowledge of the philosophy of Wolff and of Newton's physics. After the death of his father in 1746 he spent nine years as tutor in various families. In 1755 he returned to Königsberg, and there he spent the remainder of his life. From 1755 to 1770 he was Privatdozent (unsalaried professor) at the University of Königsberg. In 1770 he was appointed professor of philosophy, a position which he held until 1797.

It is usual to distinguish two periods of Kant's literary activity. The first, the pre-critical period, extends from 1747 to 1781, the date of the epoch-making "Kritik der reinen Vernunft"; the second, the critical period, extends from 1781 to 1794.

Kant's first book, which was published in 1747, was entitled "Gedanken von der wahren Schatzung der lebendigen Kräfte" (Thoughts on the True Estimation of Living Forces). In 1775 he published his doctor's dissertation, "On Fire" (De Igne), and the work "Principiorum Primorum Cognitionis Metaphysicae Nova Dilucidatio" (A New Explanation of the First Principles of Metaphysical Knowledge), by which he qualified for the position of Privatdozent. Besides these, in which he expounded and defended the current philosophy of Wolff, he published other treatises in which he applied that philosophy to problems of mathematics and physics. In 1770 appeared the work "De Mundi Sensibilis atque Intelligibilis Formis et Principiis" (On the Forms and Principles of the Sensible and Intelligible World), in which he shows for the first time a tendency to adopt an independent system of philosophy. The years from 1770 to 1780 were spent, as Kant himself tells us, in the preparation of the "Critique of Pure Reason".

The first work of Kant in which he appears as an exponent of transcendental criticism is the "Critique of Pure Reason" (Kritik der reinen Vernunft), which appeared in 1781. A second edition was published in 1787. In 1785 appeared the "Foundation for the Metaphysics of Ethics" (Grundlegung zur Metaphysik der Sitten). Then came a succession of critical works, the most important of which are the "Critique of Practical Reason" (Kritik der praktischen Vernunft), the "Critique of Judgment" (Kritik der Urtheilskraft, 1790), and "Religion within the Limits of Mere Reason" (Religion innerhalb der Grenzen der blossen Vernunft, 1793). The best editions of Kant's complete works are Hartenstein's second edition (8 vols., Leipzig, 1867-69), Rosenkranz and Schubert's (12 vols., Leipzig, 1834-42), and the edition which is being published by the Academy of Sciences of Berlin (Kants gesammelte Schriften, herausg. von der königlich preussischen Akademie der Wissenschaften, Berlin, 1902-).

During the period of his academic career, extending from 1747 to 1781, Kant, as has been said, taught the philosophy then prevalent in Germany, which was Wolff's modified form of dogmatic rationalism. That is to say, he made psychological experience to be the basis of all metaphysical truth, rejected skepticism, and judged all knowledge by the test of reason. Towards the end of that period, however, he began to question the solidity of the psychological basis of metaphysics, and ended by losing all faith in the validity and value of metaphysical reasoning. The apparent contradictions which he found to exist in the physical sciences, and the conclusions which Hume had reached in his analysis of the principle of causation, "awoke Kant from his dogmatic slumber" and brought home to him the necessity of reviewing or criticizing all human experience for the purpose of restoring the physical sciences to a degree of certitude which they rightly claim, and also for the purpose of placing on an unshakable foundation the metaphysical truths which Hume's skeptical phenomenalism had overthrown. The old rational dogmatism had, he now considered, laid too much emphasis on the a priori elements of knowledge; on the other hand, as he now for the first time realized, the empirical philosophy of Hume had gone too far when it reduced all truth to empirical or a posteriori elements. Kant, therefore, proposes to pass all knowledge in review in order to determine how much of it is to be assigned to the a priori, and how much to the a posteriori factors, if we may so designate them, of knowledge. As he himself says, his purpose is to "deduce" the a priori or transcendental, forms of thought. Hence, his philosophy is essentially a "criticism", because it is an examination of knowledge, and "transcendental", because its purpose in examining knowledge is to determine the a priori, or transcendental, forms. Kant himself was wont to say that the business of philosophy is to answer three questions: What can I know? What ought I to do? What may I hope for? He considered, however, that the answer to the second and third depends on the answer to the first; our duty and our destiny can be determined only after a thorough study of human knowledge.

It will be found most convenient to divide the study of Kant's critical philosophy into three portions, corresponding to the doctrines contained in his three "Critiques". We shall, therefore, take up successively (1) the doctrines of the "Critique of Pure Reason"; (2) the doctrines of the "Critique of Practical Reason"; (3) the doctrines of the "Critique of the Faculty of Judgment".

"Critique of Pure Reason"

In accordance with his purpose to examine all knowledge in order to find what is and what is not a priori, or transcendental, that is anterior to experience, or independent of experience, Kant proceeds in the "Critique of Pure Reason" to inquire into the a priori forms of (a) sensation, (b) judgment, and (c) reasoning.

A. Sensation

The first thing that Kant does in his study of knowledge is to distinguish between the material, or content, and the form, of sensation. The material of our sense-knowledge comes from experience. The form, however, is not derived through the senses, but is imposed on the material, or content, by the mind, in order to render the material, or content, universal and necessary. The form is, therefore, a priori; it is independent of experience. The most important forms of sense-knowledge, the conditions, in fact, of all sensation, are space and time. Not only, then, are space and time mental entities in the sense that they are elaborated by the mind out of the data of experience; they are strictly subjective, purely mental, and have no objective entity, except in so far as they are applied to the external world by the mind.

Because of what is to follow, it is important to ask at this point: Do the a priori forms of sensation, since they admittedly enhance the value of sense-knowledge by rendering it universal and necessary, extend the domain of sense-knowledge, and carry us outside the narrow confines of the material, or data, of the senses? Kant holds that they do not. They affect knowledge, so to speak, qualitatively, not quantitatively. Now, the data of sensation represent only the appearances (Erscheinungen) of things; therefore all sensation is confined to a knowledge of appearances. Sense-knowledge cannot penetrate to the noumenon, the reality of the thing (Ding-an-sich).

B. Judgment

(b) Taking up now the knowledge which we acquire by means of the understanding (Verstand), Kant finds that thought in the strict sense begins with judgment. As in the case of sense-knowledge, he distinguishes here the content and the form. The content of judgment, or in other words, that which the understanding joins together in the act of judgment, can be nothing but the sense-intuitions, which take place, as has been said, by the imposition of the forms of space and time on the data of sensation. Sometimes the sense-intuitions (subject and predicate) are joined together in a manner that evidently implies contingency and particularity. An example would be the judgment, "This table is square." With judgments of this kind the philosopher is not much concerned. He is interested rather in judgments such as "All the sides of a square are equal", in which the relation affirmed to exist between the subject and the predicate is necessary and universal. With regard to these, Kant's first remark is that their necessity and universality must be a priori. That nothing which is universal and necessary can come from experience is axiomatic with him. There must, then, be forms of judgment, as there are forms of sensation, which are imposed by the understanding, which do not come from experience at all, but are a priori. These forms of judgment are the categories. It is hardly necessary to call attention to the contrast between the Kantian categories and the Aristotelean. The difference is fundamental, a difference in nature, purpose, function, and effect. The important point for the student of Kant is to determine the function of the categories. They serve to confer universality and necessity on our judgments. They serve, moreover, to bring diverse sense-intuitions under some degree of unity. But they do not extend our knowledge. For while representations (or intuitions) without the categories would be blind, the categories without representative, or intuitional, content, would be empty. We are still within the narrow circle of knowledge covered by our sense-experience. Space and time do not widen that circle; neither do the categories. The knowledge, therefore, which we acquire by the understanding is confined to the appearances of things, and does not extend to the noumenal reality, the Ding-an-Sich.

It is necessary at this point to explain what Kant means by the "synthetic a priori" judgments. The Aristotelean philosophers distinguished two kinds of judgments, namely, synthetic judgments, which are the result of a "putting-together" (synthesis) of the facts, or data, of experience, and analytic judgments, which are the result of a "taking-apart" (analysis) of the subject and predicate, without immediate reference to experience. Thus, "This table is round" is a synthetic judgment; "All the radii of a circle are equal" is an analytic judgment. Now, according to the Aristoteleans, all synthetic judgments are a posteriori, because they are dependent on experience, and all analytic judgments are a priori, because the bond, or nexus, in them is perceived without appeal to experience. This classification does not satisfy Kant. He contends that analytic judgments of the kind referred to do not advance knowledge at all, since they always "remain within the concepts [subject and predicate] and make no advance beyond the data of the concepts". At the same time he contends that the synthetic judgments of the Aristoteleans have no scientific value, since, coming as they do from experience, they must be contingent and particular. Therefore he proposes to introduce a third class, namely, synthetic a priori judgments, which are synthetic because the content of them is supplied by a synthesis of the facts of experience, and a priori, because the form of universality and necessity is imposed on them by the understanding independently of experience. An example would be, according to Kant, "Every effect must have a cause." Our concepts of "effect" and "cause" are supplied by experience; but the universality and necessity of principle are derived from the a priori endowment of the mind. The Aristoteleans answer, and rightly, that the so-called synthetic a priori judgments are all analytic.

C. Reasoning

In the third place, Kant's "Critique of Pure Reason" is occupied with the reasoning faculty (Vernunft). Here "ideas" play a role similar to that played in sensation and judgment by space and time and the categories, respectively. Examining the reasoning faculty, Kant finds that it has three distinct operations, namely, categorical, hypothetical, and disjunctive reasoning. To these, he says, correspond the three "ideas", the idea of the soul as thinking subject (psychological idea), the idea of matter as the totality of phenomena (cosmological idea), and the idea of God as the supreme condition of all reality (theological idea). He first takes up the idea of the soul, and, examining the course of reasoning of the psychologist who teaches the substantiality, immateriality, and immortality of the human soul, he pronounces that line of philosophical thought to be fallacious, because it starts with the false supposition that we can have an intuitive knowledge of the soul as the substantial subject of conscious states. This, he claims, is an erroneous supposition, for, while we can and do know our conscious states, we cannot know the subject of them. Rational psychology, then, makes a wrong start; its way is full of contradictions; it does not conclusively establish the immortality of the soul. Next, Kant subjects the cosmological idea to a similar analysis. He finds that as soon as we begin to predicate anything concerning the ultimate nature of matter we fall into a whole series of contradictions, which he calls "antinomies". Thus, the propositions, "Matter has a beginning", "The world was created", are apparently no more true than their contradictories, "Matter is eternal", "The world is uncreated." To every thesis regarding the ultimate nature of the material universe an equally plausible antithesis may be opposed. The conclusion is that by pure reason alone we cannot attain a knowledge of the nature of the material universe. Finally, Kant takes up the theological idea, the idea of God, and criticizes the methods and arguments of rational theology. The speculative basis of our belief in the existence of God is unsound he says, because the proofs brought forward to support it are not conclusive. St. Anselm's ontological argument tries to establish an existential proposition without reference to experience; it confounds the order of things with the order of ideas. The cosmological argument carries the principle of causality beyond the world of sense-experience, where alone it is valid. And the physico-theological argument from design, while it may prove the existence of an intelligent designer, cannot establish the existence of a Supreme Being. Kant, of course, does not deny the existence of God, neither does he deny the immortality of the soul or the ultimate reality of matter. His aim is to show that the three ideas, or, in other words, speculative reasoning concerning the soul, the universe, and God, do not add to our knowledge. But, although the ideas do not extend our experience, they regulate it. The best way to think about our conscious states is to represent them as inhering in a substantial subject, about which, however, we can know nothing. The best way to think of the external world is to represent it as a multiplicity of appearances, the ground of which is an unknowable material something; and the best way to organize and systematize all our knowledge of reality is to represent everything as springing from one source, governed by one law, and tending towards one end, the law, the source, and the end being an unknown and (speculatively) unknowable God. It is very easy to see how this negative phase of Kant's philosophy affected the subsequent course of philosophic thought in Europe. The conclusions of the first "Critique" are the premises of contemporary Agnosticism. We can know nothing except the appearances of things; the senses reach only phenomena; judgment does not go any deeper than the senses, so far as the external world is concerned; science and philosophy fail utterly in the effort to reach a knowledge of substance (noumenon), or essence, and the attempts of metaphysics to teach us what the soul is, what matter is, what God is, have failed and are doomed to inevitable failure. These are the conclusions which Kant reaches in the "Critique of Pure Reason"; they are the assumptions of the Agnostic and of the Neo-Kantian opponent of Scholasticism.

"Critique of Practical Reason"

Kant, it has often been said, tore down in order to build up. What he took away in the first "Critique" he gave back in the second. In the "Critique of Pure Reason" he showed that the truths which have always been considered the most important in the whole range of human knowledge have no foundation in metaphysical, that is, purely speculative, reasoning. In the "Critique of Practical Reasoning" he aims at showing that these truths rest on a solid moral basis, and are thus placed above all speculative contention and the clamour of metaphysical dispute. He has overthrown the imposing edifice which Cartesian dogmatism had built on the foundation "I think"; he now sets about the task of rebuilding the temple of truth on the foundation "I ought." The moral law is supreme. In point of certainty, it is superior to any deliverance of the purely speculative consciousness; I am more certain that "I ought" than I am that "I am glad", "I am cold", etc. In point of insistence, it is superior to any consideration of interest, pleasure or happiness; I can forego what is for my interest, I can set other considerations above pleasure and happiness, but if my conscience tells me that "I ought" to do something, nothing can gainsay the voice of conscience, though, of course, I am free to obey or disobey. This, then, is the one unshakable foundation of all moral, spiritual, and higher intellectual truth. The first peculiarity of the moral law is that it is universal and necessary. When conscience declares that it is wrong to tell a lie, the voice is not merely intended for here and now, not for "just this once", but for all time and for all space; it is valid always and everywhere. This quality of universality and necessity shows at once that the moral law has no foundation in pleasure, happiness, the perfection of self, or a so-called moral sense. It is its own foundation. Its voice reaches conscience immediately, commands unconditionally, and need give no reason for its behests. It is not, so to speak, a constitutional monarch amenable to reason, judgment, or any other faculty. It exacts unconditional, and in a sense unreasoned obedience. Hence the "hollow voice" of the moral law is called by Kant "the categorical imperative". This celebrated phrase means merely that the moral law is a command (imperative), not a form of advice or invitation to act or not to act; and it is an unconditional (categorical) command, not a command in the hypothetical mood, such as "If you wish to be a clergyman you must study theology." One should not, however, overlook the peculiarly empty character of the categorical imperative. Only in its most universal "hollow" utterances does it possess those qualities which render it unique in human experience. But as soon as the contingent data, or contents of a specific moral precept, are presented to it, it imposes its universality and necessity on them and lifts them to its own level. The contents may have been good, but they could not have been absolutely good; for nothing is absolutely good except good will--the acceptance, that is, of the moral law.

We know the moral law not by inference, but by immediate intuition. This intuition is, as it were, the primum philosophicum. It takes the place of Descartes' primary intuition of his own thought. From it all the important truths of philosophy are deduced, the freedom of the will, the immortality of the soul, and the existence of God. The freedom of the will follows from the existence of the moral law, because the fact that "I ought" implies the fact that "I can." I know that I ought to do a certain thing, and from this I infer that I can. In the order of things, of course, freedom precedes obligation. In the order of knowledge, I infer freedom from the fact of obligation. Similarly, the immortality of the soul is implied in the moral law. The moral law demands complete fulfilment of itself in absolute human perfection. But the highest perfection that man can attain in this life is only partial or incomplete perfection, because, so long as the soul is united with the body, there is always in our nature a mixture of the corporeal with the spiritual; the striving towards holiness is accompanied by an inclination towards unholiness, and virtue implies a struggle. There must, therefore, be a life beyond the grave in which this "endless progress", as Kant calls it, will be continued. Finally, the moral law implies the existence of God. And that in two ways. The authoritative "voice" of the law implies a lawgiver. Moreover, the nature of the moral law implies that there be somewhere a good which is not only supreme, but complete, which embodies in its perfect holiness all the conditions which the moral law implies. This supreme good is God.

"Critique of the Faculty of Judgment"

Intermediate between the speculative reason, which is the faculty of knowledge, and practical reason, which is the faculty of voluntary action, is the faculty which Kant calls judgment, and which is the faculty of æsthetic appreciation. As the true is the object of knowledge, and as the good is the object of action, the beautiful and purposive is the object of judgment. By this peculiar use of the word judgment Kant places himself at once outside the ranks of the sensists, who refer all the constituents of beauty to sense-perceived qualities. He is an intellectualist in æsthetics, reducing the beautiful to elements of intellectuality. The beautiful, he teaches, is that which universally and necessarily gives disinterested pleasure, without the concept of definite design. It differs, consequently, from the agreeable and the useful. However, Kant is careful to remark that the enjoyment of the beautiful is not purely intellectual, as is the satisfaction which we experience in contemplating the perfect. The perfect appeals to the intellect alone, while the beautiful appeals also to the emotions and to the æsthetic faculty. Closely allied to the beautiful is the purposive. The same faculty, judgment, which enables us to perceive and enjoy the æsthetic aspect of nature and of art, enables us also to perceive that in the manifold variety of our experience there is evidence of purpose or design. Kant introduced in his "Critique" of the teleological judgment an important distinction between external and internal adaptation. External adaptation, he taught, exists between the organism and its environment, as, for instance, between the plant and the soil in which it grows. Internal adaptation exists among the structural parts of the organism, or between the organism and its function. The former, he believed, could be explained by merely mechanical causes, but the latter necessitates the introduction of the concept of final cause. Organisms act as though they were produced by a cause which had a purpose in view. We cannot clearly demonstrate that purpose. The teleological concept is, therefore, like the "ideas" (the soul, the world, God) not constitutive of our experience but regulative of it. The highest use of the æsthetic faculty is the realization of the beautiful and the purposive as symbols of moral good. What speculative reason fails to find in nature, namely, a beautiful, purposive order, is suggested by the æsthetic judgment and fully attained by religion, which rests on the practical reason.

Kant, as is well known, reduces religion to a system of conduct. He defines religion as "the acknowledgement that our duties are God's commandments". He describes the essence of religion as consisting in morality. Christianity is a religion and is true only in so far as it conforms to this definition. The ideal Church should be an "ethical republic"; it should discard all dogmatic definitions, accept "rational faith" as its guide in all intellectual matters, and establish the kingdom of God on earth by bringing about the reign of duty. Even the Christian law of charity must take second place to the supreme exigencies of duty. In fact, it has been remarked that Kant's idea of religion, in so far as it is at all Scriptural, is inspired more by the Old than by the New Testament. He maintains that those dogmas which Christianity holds sacred, such as the mystery of the Trinity, should be given an ethical interpretation, should, so to speak, be regarded as symbols of moral concepts and values. Thus "historical faith", he says, is the "vehicle of rational faith". For the person and character of Christ he professes the greatest admiration. Christ, he declares, was the exemplification of the highest moral perfection.

Critics and historians are not all agreed as to Kant's rank among philosophers. Some rate his contributions to philosophy so highly that they consider his doctrines to be the culmination of all that went before him. Others, on the contrary, consider that he made a false start when he assumed in his criticism of speculative reason that whatever is universal and necessary in our knowledge must come from the mind itself, and not from the world of reality outside us. These opponents of Kant consider, moreover, that while he possessed the synthetic talent which enabled him to build up a system of thought, he was lacking in the analytic quality by which the philosopher is able to observe what actually takes place in the mind. And in a thinker who reduced all philosophy to an examination of knowledge the lack of the ability to observe what actually takes place in the mind is a serious defect. But, whatever may be our estimate of Kant as a philosopher, we should not undervalue his importance. Within the limits of the philosophical sciences themselves, his thought was the starting-point for Fichte, Schelling, Hegel, and Schopenhauer; and, so far as contemporary philosophic thought in Germany is concerned, whatever of it is not Kantian takes for its distinguishing characteristic its opposition to some point of Kantian doctrine. In England the Agnostic School from Hamilton to Spencer drew its inspiration from the negative teaching of the "Critique of Pure Reason". In France the Positivism of Comte and the neo-Criticism of Renouvier had a similar origin. Kant's influence reached out beyond philosophy into various other departments of thought. In the history of the natural sciences his name is associated with that of Laplace, in the theory which accounts for the origin of the universe by a natural evolution from primitive cosmic nebula. In theology his non-dogmatic notion of religion influenced Ritschl, and his method of transforming dogmatic truth into moral inspiration finds an echo, to say the least, in the exegetical experiments of Renan and his followers.

Some philosophers and theologians have held that the objective data on which the Catholic religion is based are incapable of proof from speculative reason, but are demonstrable from practical reason, will, sentiment, or vital action. That this position is, however, dangerous, is proved by recent events. The Immanentist movement, the Vitalism of Blondel, the anti-Scholasticism of the "Annales de philosophie chretienne", and other recent tendencies towards a non-intellectual apologetic of the Faith, have their roots in Kantism, and the condemnation they have received from ecclesiastical authority shows plainly that they have no clear title to be considered a substitute for the intellectualistic apologetic which has for its ground the realism of the Scholastics.