 Salomon Maimon (1753–1800) stands as one of the most acute, original, and complicated philosophers — and certainly one of the most fascinating personalities — of the eighteenth century. By granting the principle of sufficient reason unlimited validity Maimon embraces a radical form of rationalism. His robust criteria for the validity of knowledge suggests that even Kant’s attempt to limit epistemological claims to the realm of possible experience cannot be secured without a substantial ontological commitment. Kant faces a severe choice: either adopt elements from the dogmatic, rationalist, metaphysics he set out to challenge, or accept that his system is undermined by skepticism. In revealing what he sees as the skeptical implications of rationalism, Maimon raises important objections to Kant’s critical idealism, as well as develops deep insights into the problems of experience and givenness. His ‘skeptically rationalist’ claims about the nature and limits of human cognition present a distinctive perspective on the Kantian project of transcendental idealism, as well as on central epistemological issues concerning the relation between thought and the world. The eminent German scholar, Manfred Frank recently suggested that Maimon is the “last great philosopher” about to be discovered.