The son of a saddler, Kant was born and educated in Königsberg (Kaliningrad) in East Prussia. After leaving the university he spent a number of years in private tutoring, but taking his master’s degree in 1755, he settled to teach a variety of subjects as Privatdozent. Kant’s early writings concern physics and astronomy: his Allgemeine Naturgeschichte und Theorie des Himmels (1755, trs. as Universal Natural History and Theory of the Heavens, 1969) predicted the existence of the planet Uranus, later discovered by Herschel in 1881. In 1770 he was appointed to the chair of logic and metaphysics at Königsberg. It was after this that he entered on his greatest, ‘critical’ period. His life was orderly to the point of caricature: he never left Königsberg, and never married.
The intellectual landscape in which Kant began his career was largely set by Leibniz, filtered through Wolff, who had erected a structured and orderly system out of Leibniz’s thought. Wolff believed that the principle of sufficient reason, and much resulting metaphysics, could be known a priori, although the status of this knowledge was already doubted by such men as Crusius. However, it was generally felt that ‘intuition’ afforded us knowledge and was guaranteed by God, so all was well. In his pre-critical Dreams of a Spirit-Seer (Träume eines Geistersehers, 1766), the most hostile to metaphysics of all his works, Kant treats the speculations of Crusius and Wolff as spun out of nothing, like the spiritual imaginings of Swedenborg. The first step to the critical philosophy was the Inaugural Dissertation of 1770 (Latin title: De Mundi Sensibilis atque Intelligibilis Forma et Principiis), in which for the first time Kant unveils the view that we can have a priori knowledge of space and time only because they are forms imposed by our own minds upon experience (see Copernican revolution). Space is a ‘schema, issuing by a constant law from the nature of the mind, for the coordinating of all outer sensa whatever’. The Dissertation heralds themes that were worked out for the next ten years: the subjective origin of the schema of space and time, the distinction that it creates between things as they are in themselves and things as they are to us, and the distinction between experience and thought.
The Critique of Pure Reason (Kritik der reinen Vernunft, 1781, known as the first Critique) extends these themes to cover all the categories used in thought. Its aim is to ‘assure to reason its lawful claims, and dismiss all groundless pretensions, not by despotic decrees, but in accordance with its own eternal and unalterable laws’ (Preface to the 1st edition). In answer to its guiding question (how is synthetic a priori knowledge possible?), the first part of the work isolates legitimate categories and provides them with a ‘transcendental deduction’, guaranteeing their objective applicability by showing that without them experience is itself impossible. One of Kant’s central moves is to argue that the unity of consciousness itself presupposes orderly experience, tied together in accordance with universal and necessary laws. It is this part of his work that constitutes his attempt to answer the inductive scepticism, and subjectivity about causation, left by Hume. (Kant’s famous remark that it was Hume who awoke him from his dogmatic slumbers was made in the Prologomena to any Future Metaphysics in 1783, but in fact the influence of Hume on the first Critique is quite slight.) Having established the proper authority and provenance of reason, Kant turns, in the section of the first Critique called the Dialectic, to cases where the pretensions of reason get out of hand, producing the dogmatic metaphysics that purports to establish doctrines about the nature of the self, the constitution of an independent order of space and time, and the knowability of God.
The first Critique is a preparation for the concern with practical reason that now occupied his attention. Having laid the foundations of a critical philosophy he produced the Fundamental Principles of the Metaphysic of Morals (Grundlegung zur Metaphysik der Sitten, 1785), and the Critique of Practical Reason (Kritik der praktischen Vernunft, 1788, known as the second Critique). Kant said that ‘two things move the mind with ever increasing admiration and awe, the oftener and more steadily we reflect on them: the starry heavens above and the moral law within’ (conclusion of the second Critique; the identical formula is found in St John Chrysostom and derives from the 19th Psalm). His ethics is based uncompromisingly on the search for a single supreme principle of morality, a principle moreover that has rational authority, leading rather than following the passions, and binding on all rational creatures. Every action springs from some subjective principle, or maxim, and the moral worth of an individual lies entirely in the question of whether the maxim of his or her action is respect for the law, the duty of obeying the categorical imperative. Kant’s own applications of this test forbid lying, suicide, revolution against the extant political order, solitary sex, and selling one’s hair for wig-making, but the extent to which his ethics can be disentangled from its Lutheran matrix is controversial. Kant’s restriction of moral worth to a specific kind of concern with duty has frequently seemed to downgrade normal human virtues such as benevolence, but commentators try to discern a more humane Kant behind the stern and rigorous ethic of respect for law (see apathy). Other contested components of his ethical system include the association of freedom with capacity for self-control and self-legislation, and the practical need to think in terms of a just God sustaining the moral order.
The third Critique, the Critique of Judgement (Kritik der Urteilskraft, 1790), confronts the difficulty of making aesthetic judgements objective, given that they are not made in accordance with a rule but in response to subjective pleasure. Kant connects our right to demand accord from others in such matters to a teleology or goal-directed conception of nature, an idea common to the Romanticism of the time and to which he lent his great authority (see also aesthetics). Kant’s concern with the basis of metaphysics and knowledge had not vanished, for at this time he also produced the Metaphysical Foundations of Natural Science (Metaphysische Anfangsgründe der Naturwissenschaft, 1786). This work treats the nature of motion, matter, and mass, and espouses a kind of field theory that was to become influential in 19th-century physics. Later works include Religion within the Limits of Reason Alone (Die Religion innerhalb der Grenzen der bloßen Vernunft, 1793) which got Kant into trouble with the repressive religious censorship of Frederick William II of Prussia; Perpetual Peace (Zum ewigen Frieden, 1795); and Die Metaphysik der Sitten, 1797 (trs. as The Metaphysics of Morals, but often in two different parts, The Metaphysical Principles of Right, and The Metaphysical Principles of Virtue.)
In spite of the notorious difficulty of reading Kant, made worse by his penchant for scholastic systematization and obscure terminology, his place as the greatest philosopher of the last three hundred years is well assured. He made the first decisive break with the sensationalist empiricism that prevailed in the 18th century, but without retreating to an indefensible rationalism. Whilst his confidence in the a priori and the structure of his idealism have been widely rejected, it is not too much to say that all modern epistemology, metaphysics, and even ethics, is implicitly affected by the architecture he created. The standard modern biography is by Manfred Kuehn.