Kant was the foremost thinker of the Enlightenment and one of the greatest philosophers of all time. In him were subsumed new trends that had begun with the Rationalism (stressing reason) of René Descartes and the Empiricism (stressing experience) of Francis Bacon. He thus inaugurated a new era in the development of philosophical thought.

Kant lived in the remote province where he was born for his entire life. His father, a saddler, was, according to Kant, a descendant of a Scottish immigrant, although scholars have found no basis for this claim; his mother, an uneducated German woman, was remarkable for her character and natural intelligence. Both parents were devoted followers of the Pietist branch of the Lutheran Church, which taught that religion belongs to the inner life expressed in simplicity and obedience to moral law. The influence of their pastor made it possible for Kant—the fourth of nine children, but the eldest surviving child—to obtain an education.

At the age of eight Kant entered the Pietist school that his pastor directed. This was a Latin school, and it was presumably during the eight and a half years he was there that Kant acquired his lifelong love for the Latin classics, especially for the naturalistic poet Lucretius. In 1740 he enrolled in the University of Königsberg as a theological student. But, although he attended courses in theology and even preached on a few occasions, he was principally attracted to mathematics (mathematics, philosophy of) and physics. Aided by a young professor who had studied Christian Wolff (Wolff, Christian, Freiherr (Baron) von), a systematizer of Rationalist philosophy, and who was also an enthusiast for the science of Sir Isaac Newton (Newton, Sir Isaac), Kant began reading the work of the English physicist and, in 1744, started his first book, dealing with a problem concerning kinetic forces. Though by that time he had decided to pursue an academic career, the death of his father in 1746 and his failure to obtain the post of undertutor in one of the schools attached to the university compelled him to withdraw and seek a means of supporting himself.

He found employment as a family tutor and, during the nine years that he gave to it, worked for three different families. With them he was introduced to the influential society of the city, acquired social grace, and made his farthest travels from his native city—some 60 miles (96 kilometres) away to the town of Arnsdorf. In 1755, aided by the kindness of a friend, he was able to complete his degree at the university and take up the position of Privatdozent, or lecturer.

Three dissertations that he presented on obtaining this post indicate the interest and direction of his thought at this time. In one, De Igne (On Fire), he argued that bodies operate on one another through the medium of a uniformly diffused elastic and subtle matter that is the underlying substance of both heat and light. His first teaching was in mathematics and physics, and he was never to lose his interest in scientific developments. That it was more than an amateur interest is shown by his publication within the next few years of several scientific works dealing with the different races of men, the nature of winds, the causes of earthquakes, and the general theory of the heavens.

At this period Newtonian physics was important to Kant as much for its philosophical implications as for its scientific content. A second dissertation, the Monodologia physica (1756), contrasted the Newtonian methods of thinking with those employed in the philosophy then prevailing in German universities. This was the philosophy of Gottfried Wilhelm Leibniz (Leibniz, Gottfried Wilhelm), a universal scholar, as systematized and popularized by Wolff and by Alexander Gottlieb Baumgarten (Baumgarten, Alexander Gottlieb), author of a widely used text, the Metaphysica (1739). Leibniz' works as they are now known were not fully available to these writers; and the Leibnizian philosophy that they presented was extravagantly Rationalistic, abstract, and cut-and-dried. It nevertheless remained a powerful force, and the main efforts of independent thinkers in Germany at the time were devoted to examining Leibniz's ideas.

In a third dissertation, Principiorum Primorum Cognitionis Metaphysicae Nova Dilucidato (1755), on the first principles of metaphysics, Kant analyzed especially the principle of sufficient reason (sufficient reason, principle of), which, in Wolff's formulation, asserts that for everything there is a sufficient reason why it should be rather than not be. Although critical, Kant was cautious and still a long way from challenging the assumptions of Leibnizian metaphysics.

During the 15 years that he spent as a Privatdozent, Kant's renown as a teacher and writer steadily increased. Soon he was lecturing on many subjects other than physics and mathematics—including logic, metaphysics, and moral philosophy. He even lectured on fireworks and fortifications and every summer for 30 years taught a popular course on physical geography. He enjoyed great success as a lecturer; his lecturing style, which differed markedly from that of his books, was humorous and vivid, enlivened by many examples from his reading in English and French literature, and in travel and geography, science and philosophy.

Although he twice failed to obtain a professorship at Königsberg, he refused to accept offers that would have taken him elsewhere—including the professorship of poetry at Berlin that would have brought greater prestige. He preferred the peace and quiet of his native city in which to develop and mature his own philosophy.

During the 1760s he became increasingly critical of Leibnizianism. According to one of his students, Kant was then attacking Leibniz, Wolff, and Baumgarten, was a declared follower of Newton, and expressed great admiration for the moral philosophy of the Romanticist Jean-Jacques Rousseau.

His principal work of this period was Untersuchung über die Deutlichkeit der Grundsätze der natürlichen Theologie und der Moral (1764; “An Inquiry into the Distinctness of the Fundamental Principles of Natural Theology and Morals”). In this work he attacked the claim of Leibnizian philosophy that philosophy should model itself on mathematics and aim at constructing a chain of demonstrated truths based on self-evident premises. Kant argued that mathematics proceeds from definitions that are arbitrary, by means of operations that are clearly and sharply defined, upon concepts that can be exhibited in concrete form. In contrast with this method, he argued that philosophy must begin with concepts that are already given, “though confusedly or insufficiently determined,” so that philosophers cannot begin with definitions without thereby shutting themselves up within a circle of words. Philosophy cannot, like mathematics, proceed synthetically; it must analyze and clarify. The importance of the moral order, which he had learned from Rousseau, reinforced the conviction received from his study of Newton that a synthetic philosophy is empty and false.

Besides attacking the methods of the Leibnizians, he also began criticizing their leading ideas. In an essay Versuch, den Begriff der negativen Grössen in die Weltweisheit ein-zuführen (1763), he argued that physical opposition as encountered in things cannot be reduced to logical (logic) contradiction, in which the same predicate is both affirmed and denied, and, hence, that it is pointless to reduce causality to the logical relation of antecedent and consequent. In an essay of the same year, Der einzig mögliche Beweisgrund zu einer Demonstration des Daseyns Gottes, he sharply criticized the Leibnizian concept of Being by charging that the so-called ontological argument, which would prove the existence of God by logic alone, is fallacious because it confuses existential with attributive statements: existence, he declared, is not a predicate of attribution. Moreover, with regard to the nature of space, Kant sided with Newton in his confrontation with Leibniz. Leibniz' view that space is “an order of co-existences” and that spatial differences can be stated in conceptual terms, he concluded to be untenable.

Some indication of a possible alternative of Kant's own to the Leibnizian position can be gathered from his curious Träume eines Geistersehers erläutert durch Träume der Metaphysik (1766). This work is an examination of the whole notion of a world of spirits, in the context of an inquiry into the spiritualist (spiritualism) claims of Emanuel Swedenborg (Swedenborg, Emanuel), a scientist and biblical scholar. Kant's position at first seems to have been completely skeptical, and the influence of the Scottish Skeptic David Hume (Hume, David) is more apparent here than in any previous work; it was Hume, he later claimed, who first awoke him from his dogmatic slumbers. Yet Kant was not so much arguing that the notion of a world of spirits is illusory as insisting that men have no insight into the nature of such a world, a conclusion that has devastating implications for metaphysics as the Leibnizians conceived it. Metaphysicians can dream as well as spiritualists, but this is not to say that their dreams are necessarily empty; there are already hints that moral experience can give content to the ideal of an “intelligible world.” Rousseau thus acted upon Kant here as a counterinfluence to Hume.

Finally, in 1770, after serving for 15 years as a Privatdozent, Kant was appointed to the chair of logic and metaphysics, a position in which he remained active until a few years before his death. In this period—usually called his critical period, because in it he wrote his great Critiques—he published an astounding series of original works on a wide variety of topics, in which he elaborated and expounded his philosophy.

The Inaugural Dissertation of 1770 that he delivered on assuming his new position already contained many of the important elements of his mature philosophy. As indicated in its title, De Mundi Sensibilis atque Intelligibilis Forma et Principiis: Dissertatio, the implicit dualism of the Träume is made explicit; and it is made so on the basis of a wholly un-Leibnizian interpretation of the distinction between sense and understanding. Sense is not, as Leibniz had supposed, a confused form of thinking but a source of knowledge in its own right, although the objects so known are still only “appearances”—the term that Leibniz also used. They are appearances because all sensing is conditioned by the presence, in sensibility, of the forms of time and space, which are not objective characteristics or frameworks of things but “pure intuitions.” But though all knowledge of things sensible is thus of phenomena (phenomenon), it does not follow that nothing is known of things as they are in themselves. Certainly, man has no intuition, or direct insight, into an intelligible world; but the presence in him of certain “pure intellectual concepts, such as those of possibility, existence, necessity, substance, cause, enables him to have some descriptive knowledge of it. By means of these concepts he can arrive at an exemplar that provides him with “the common measure of all other things as far as real.” This exemplar gives man an idea of perfection for both the theoretical and practical orders: in the first, it is that of the Supreme Being, God; in the latter, that of moral perfection.

After the Dissertation, Kant published virtually nothing for 11 years. Yet, in submitting the Dissertation to a friend at the time of its publication, he wrote:

In 1781 the Kritik der reinen Vernunft (spelled “Critik” in the first edition; Critique of Pure Reason) was published, followed for the next nine years by great and original works that in a short time brought a revolution in philosophical thought and established the new direction in which it was to go in the years to come.

The Critique of Pure Reason was the result of some 10 years of thinking and meditation. Yet, even so, Kant published the first edition only reluctantly after many postponements; for although convinced of the truth of its doctrine, he was uncertain and doubtful about its exposition. His misgivings proved well-founded, and Kant complained that interpreters and critics of the work were badly misunderstanding it. To correct these wrong interpretations of his thought he wrote the Prolegomena zu einer jeden künftigen Metaphysik die als Wissenschaft wird auftreten können (1783) and brought out a second and revised edition of the first “critique” in 1787. Controversy still continues regarding the merits of the two editions: readers with a preference for an Idealistic interpretation usually prefer the first edition, whereas those with a Realistic view adhere to the second. But with regard to difficulty and ease of reading and understanding, it is generally agreed that there is little to choose between them. Anyone on first opening either book finds it overwhelmingly difficult and impenetrably obscure.

The cause for this difficulty can be traced in part to the works that Kant took as his models for philosophical writing. He was the first great modern philosopher to spend all of his time and efforts as a university professor of the subject. Regulations required that in all lecturing a certain set of books be used, with the result that all of Kant's teaching in philosophy had been based on such handbooks as those of Wolff and Baumgarten, which abounded in technical jargon, artificial and schematic divisions, and great claims to completeness. Following their example, Kant accordingly provided a highly artificial, rigid, and by no means immediately illuminating scaffolding for all three of his Critiques.

The Critique of Pure Reason, after an introduction, is divided into two parts, of very different lengths: A “Transcendental Doctrine (transcendental idealism) of Elements,” running to almost 400 pages in a typical edition, followed by a “Transcendental Doctrine of Method,” which reaches scarcely 80 pages. The “. . . Elements” deals with the sources of human knowledge, whereas the “. . . Method” draws up a methodology for the use of “pure reason” and its a priori (a priori knowledge) ideas. Both are “transcendental,” in that they are presumed to analyze the roots of all knowledge and the conditions of all possible experience. The “Elements” is divided, in turn, into a “Transcendental Aesthetic,” a “Transcendental Analytic,” and a “Transcendental Dialectic.”

The simplest way of describing the contents of the Critique is to say that it is a treatise about metaphysics: it seeks to show the impossibility of one sort of metaphysics and to lay the foundations for another. The Leibnizian (Leibniz, Gottfried Wilhelm) metaphysics, the object of his attack, is criticized for assuming that the human mind can arrive, by pure thought, at truths about entities, which, by their very nature, can never be objects of experience, such as God, human freedom, and immortality. Kant maintained, however, that the mind has no such power and that the vaunted metaphysics is thus a sham.

As Kant saw it, the problem of metaphysics, as indeed of any science, is to explain how, on the one hand, its principles can be necessary and universal (such being a condition for any knowledge that is scientific) and yet, on the other hand, involve also a knowledge of the real and so provide the investigator with the possibility of more knowledge than is analytically contained in what he already knows; i.e., than is implicit in the meaning alone. To meet these two conditions, Kant maintained, knowledge must rest on judgments that are a priori, for it is only as they are separate from the contingencies of experience that they could be necessary and yet also synthetic; i.e., so that the predicate term contains something more than is analytically contained in the subject. Thus, for example, the proposition that all bodies are extended is not synthetic but analytic because the notion of extension is contained in the very notion of body; whereas the proposition that all bodies are heavy is synthetic because weight supposes, in addition to the notion of body, that of bodies in relation to one another. Hence, the basic problem, as Kant formulated it, is to determine “How 【i.e., under what conditions】 are synthetic a priori judgments possible?”

This problem arises, according to Kant, in three fields, viz., in mathematics, physics, and metaphysics; and the three main divisions of the first part of the Critique deal respectively with these. In the “Transcendental Aesthetic,” Kant argued that mathematics (mathematics, philosophy of) necessarily deals with space and time and then claimed that these are both a priori forms of human sensibility that condition whatever is apprehended through the senses. In the “Transcendental Analytic,” the most crucial as well as the most difficult part of the book, he maintained that physics is a priori and synthetic because in its ordering of experience it uses concepts of a special sort. These concepts—“categories (category),” he called them—are not so much read out of experience as read into it and, hence, are a priori, or pure, as opposed to empirical. But they differ from empirical concepts in something more than their origin: their whole role in knowledge is different; for, whereas empirical concepts serve to correlate particular experiences and so to bring out in a detailed way how experience is ordered, the categories have the function of prescribing the general form that this detailed order must take. They belong, as it were, to the very framework of knowledge. But although they are indispensable for objective knowledge, the sole knowledge that the categories can yield is of objects of possible experience; they yield valid and real knowledge only when they are ordering what is given through sense in space and time.

In the “Transcendental Dialectic” Kant turned to consideration of a priori synthetic judgments in metaphysics. Here, he claimed, the situation is just the reverse from what it was in mathematics and physics. Metaphysics cuts itself off from sense experience in attempting to go beyond it and, for this very reason, fails to attain a single true a priori synthetic judgment. To justify this claim, Kant analyzed the use that metaphysics makes of the concept of the unconditioned. Reason, according to Kant, seeks for the unconditioned or absolute in three distinct spheres: (1) in philosophical psychology it seeks for an absolute subject of knowledge; (2) in the sphere of cosmology, it seeks for an absolute beginning of things in time, for an absolute limit to them in space, and for an absolute limit to their divisibility; and (3) in the sphere of theology, it seeks for an absolute condition for all things. In each case, Kant claimed to show that the attempt is doomed to failure by leading to an antinomy in which equally good reasons can be given for both the affirmative and the negative position. The metaphysical “sciences” of rational psychology, rational cosmology, and natural theology, familiar to Kant from the text of Baumgarten, on which he had to comment in his lectures, thus turn out to be without foundation.

With this work, Kant proudly asserted that he had accomplished a Copernican revolution in philosophy. Just as the founder of modern astronomy, Nicolaus Copernicus, had explained the apparent movements of the stars by ascribing them partly to the movement of the observers, so Kant had accounted for the application of the mind's a priori principles to objects by demonstrating that the objects conform to the mind: in knowing, it is not the mind that conforms to things but instead things that conform to the mind.

Because of his insistence on the need for an empirical component in knowledge and his antipathy to speculative metaphysics, Kant is sometimes presented as a Positivist before his time; and his attack upon metaphysics was held by many in his own day to bring both religion and morality down with it. Such, however, was certainly far from Kant's intention. Not only did he propose to put metaphysics “on the sure path of science,” he was prepared also to say that he “inevitably” believed in the existence of God and in a future life. It is also true that his original conception of his critical philosophy anticipated the preparation of a critique of moral philosophy. The Kritik der praktischen Vernunft (1788, spelled “Critik” and “practischen”; Critique of Practical Reason), the result of this intention, is the standard source book for his ethical doctrines. The earlier Grundlegung zur Metaphysik der Sitten (1785) is a shorter and, despite its title, more readily comprehensible treatment of the same general topic. Both differ from Die Metaphysik der Sitten (1797) in that they deal with pure ethics and try to elucidate basic principles; whereas the later work is concerned with applying what they establish in the concrete, a process that involved the consideration of virtues and vices and the foundations of law and politics.

There are many points of similarity between Kant's ethics and his epistemology, or theory of knowledge. He used the same scaffolding for both—a “Doctrine of Elements,” including an “Analytic” and a “Dialectic,” followed by a “Methodology”; but the second Critique is far shorter and much less complicated. Just as the distinction between sense and intelligence was fundamental for the former, so is that between the inclinations and moral reason for the latter. And just as the nature of the human cognitive situation was elucidated in the first Critique by reference to the hypothetical notion of an intuitive understanding, so is that of the human moral situation clarified by reference to the notion of a “holy will.” For a will of this kind there would be no distinction between reason and inclination; a being possessed of a holy will would always act as it ought. It would not, however, have the concepts of duty and moral obligation, which enter only when reason and desire find themselves opposed. In the case of human beings, the opposition is continuous, for man is at the same time both flesh and spirit; it is here that the influence of Kant's religious background is most prominent. Hence, the moral life is a continuing struggle in which morality appears to the potential delinquent in the form of a law that demands to be obeyed for its own sake—a law, however, the commands of which are not issued by some alien authority but represent the voice of reason, which the moral subject can recognize as his own.

In the “Dialectic,” Kant took up again the ideas of God, freedom, and immortality. Dismissed in the first Critique as objects that men can never know because they transcend human sense experience, he now argued that they are essential postulates for the moral life. Though not reachable in metaphysics, they are absolutely essential for moral philosophy.

Kant is often described as an ethical Rationalist, and the description is not wholly inappropriate. He never espoused, however, the radical Rationalism of some of his contemporaries nor of more recent philosophers for whom reason is held to have direct insight into a world of values or the power to intuit the rightness of this or that moral principle. Thus, practical, like theoretical, reason was for him formal rather than material—a framework of formative principles rather than a content of actual rules. This is why he put such stress on his first formulation of the categorical imperative: “Act only on that maxim through which you can at the same time will that it should become a universal law.” Lacking any insight into the moral realm, men can only ask themselves whether what they are proposing to do has the formal character of law—the character, namely, of being the same for all persons similarly circumstanced.

The Kritik der Urteilskraft (1790: spelled “Critik”)—one of the most original and instructive of all of Kant's writings—was not foreseen in his original conception of the critical philosophy. Thus it is perhaps best regarded as a series of appendixes to the other two Critiques. The work falls into two main parts, called respectively “Critique of Aesthetic Judgment” and “Critique of Teleological Judgment (teleology).” In the first of these, after an introduction in which he discussed “logical purposiveness,” he analyzed the notion of “aesthetic purposiveness (aesthetics)” in judgments that ascribe beauty to something. Such a judgment, according to him, unlike a mere expression of taste, lays claim to general validity; yet it cannot be said to be cognitive because it rests on feeling, not on argument. The explanation lies in the fact that, when a person contemplates an object and finds it beautiful, there is a certain harmony between his imagination and his understanding, of which he is aware from the immediate delight that he takes in the object. Imagination grasps the object and yet is not restricted to any definite concept; whereas a person imputes the delight that he feels to others because it springs from the free play of his cognitive faculties, which are the same in all men.

In the second part, Kant turned to consider teleology in nature as it is posed by the existence in organic bodies of things of which the parts are reciprocally means and ends to each other. In dealing with these bodies, one cannot be content with merely mechanical principles. Yet if mechanism is abandoned and the notion of a purpose or end of nature is taken literally, this seems to imply that the things to which it applies must be the work of some supernatural designer; but this would mean a passing from the sensible to the suprasensible, a step proved in the first Critique to be impossible. Kant answered this objection by admitting that teleological language cannot be avoided in taking account of natural phenomena; but it must be understood as meaning only that organisms must be thought of “as if” they were the product of design, and that is by no means the same as saying that they are deliberately produced.

The critical philosophy was soon being taught in every important German-speaking university, and young men flocked to Königsberg as a shrine of philosophy. In some cases, the Prussian government even undertook the expense of their support. Kant came to be consulted as an oracle on all kinds of questions, including such subjects as the lawfulness of vaccination. Such homage did not interrupt Kant's regular habits. Scarcely five feet tall, with a deformed chest, and suffering from weak health, he maintained throughout his life a severe regimen. It was arranged with such regularity that people set their clocks according to his daily walk along the street named for him, “The Philosopher's Walk.” Until old age prevented him, he is said to have missed this regular appearance only on the occasion when Rousseau's Émile so engrossed him that for several days he stayed at home.

With the publication of the third Critique, Kant's main philosophical work was done. From 1790 his health began to decline seriously. He still had many literary projects but found it impossible to write more than a few hours a day. The writings that he then completed consist partly of an elaboration of subjects not previously treated in any detail, partly of replies to criticisms and to the clarification of misunderstandings. With the publication in 1793 of his work Die Religion innerhalb der Grenzen der blossen Vernunft, Kant became involved in a dispute with Prussian authorities on the right to express religious opinions. The book was found to be altogether too Rationalistic for orthordox taste; he was charged with misusing his philosophy to the “distortion and depreciation of many leading and fundamental doctrines of sacred Scripture and Christianity” and was required by the government not to lecture or write anything further on religious subjects. Kant agreed but privately interpreted the ban as a personal promise to the King, from which he felt himself to be released on the latter's death in 1797. At any rate, he returned to the forbidden subject in his last major essay, Der Streit der Fakultäten (1798; “The Conflict of the Faculties”).

The large work at which he laboured until his death—the fragments of which fill the two final volumes of the great Berlin edition of his works—was evidently intended to be a major contribution to his critical philosophy. What remains, however, is not so much an unfinished work as a series of notes for a work that was never written. Its original title was Übergang von den metaphysische Anfangsgründe der Naturwissenschaft zur Physik (“Transition from the Metaphysical Foundations of Natural Science to Physics”), and it may have been his intention to carry further the argument advanced in the Metaphysische Anfangsgründe der Naturwissenschaft (1786) by showing that it is possible to construct a priori not merely the general outline of a science of nature but a good many of its details as well. But judging from the extant fragments, however numerous they are, it remains conjectural whether its completion would have constituted a major addition to his philosophy and its reputation.

After a gradual decline that was painful to his friends as well as to himself, Kant died in Königsberg, February 12, 1804. His last words were “Es ist gut” (“It is good”). His tomb in the cathedral was inscribed with the words (in German) “The starry heavens above me and the moral law within me,” the two things that he declared in the conclusion of the second Critique “fill the mind with ever new and increasing admiration and awe, the oftener and the more steadily we reflect on.”