(born April 22, 1724, Knigsberg, Prussiadied Feb. 12, 1804, Knigsberg) German philosopher, one of the foremost thinkers of the Enlightenment. The son of a saddler, he studied at the university in Knigsberg and taught there as privatdocent (175570) and later as professor of logic and metaphysics (177097). His life was uneventful. His Critique of Pure Reason (1781) discusses the nature of knowledge in mathematics and physics and demonstrates the impossibility of knowledge in metaphysics as it was traditionally conceived. Kant argued that the propositions of mathematics and physics, but not those of metaphysics, are synthetic a priori, in the sense that they are about objects of possible experience (synthetic) but at the same time knowable prior to, or independently of, experience (a priori), thus making them also necessarily true, rather than merely contingently true ( necessity). Mathematics is synthetic and a priori because it deals with space and time, both of which are forms of human sensibility that condition whatever is apprehended through the senses. Similarly, physics is synthetic and a priori because in its ordering of experience it uses concepts (categories) whose function is to prescribe the general form that sensible experience must take. Metaphysics in the traditional sense, understood as knowledge of the existence of God, the freedom of the will, and the immortality of the soul, is impossible, because these questions transcend any possible sense experience. But though they cannot be objects of knowledge, they are nevertheless justified as essential postulates of a moral life. Kant's ethics, which he expounded in the Critique of Practical Reason (1788) and the earlier Groundwork of the Metaphysics of Morals (1785), was based on the principle known as the categorical imperative, one formulation of which is, Act only on that maxim through which you can at