Immanuel Kant revolutionized philosophy. He spent the first half of his career as a prominent rationalist in the tradition of Gottfried Leibniz, before his mid-life encounter with the empiricism of David Hume spurred him to reconsider his most basic beliefs. Kant subsequently undertook a thorough “critique” of reason, in order to specify its capacities and limitations. The result was a truly novel philosophical position, transcendental idealism, with which Kant claimed to save the possibility of knowledge from Hume's scepticism, and to establish the reality of human freedom and moral agency.

Kant's achievements transformed philosophy immediately and irrevocably. The initial consequence was the German idealism of Fichte, Schelling and Hegel. Subsequent movements indebted to Kant (if sometimes primarily as an inspiration for criticism) included Romanticism, naturalism and existentialism. Anglo-American philosophy, which early in the twentieth century defined itself in sharp distinction from the German tradition, has more recently incorporated Kantian insights in every domain. A thorough understanding of Kant is thus indispensable to contemporary philosophers of every stripe.

The significance of Kant's thought is matched by its complexity. Kant's revolutionary ideas are systematically interconnected and presented in a forbidding technical vocabulary, which makes his main works especially difficult for the uninitiated. The systematic and technical character of Kant's philosophy also offers, however, a means by which to understand it. A careful and patient investigation of the key concepts that structure Kant's philosophical project gradually leads to comprehension of the whole.