 Every reader of the Critique of Pure Reason knows that Kant glosses his philosophical project in that book as a complete and systematic answer to the question, “how are synthetic a priori judgments possible?” (B19). Also every reader of the first Critique knows that Kant asserts the existence of synthetic a priori judgments in mathematics, physics, and metaphysics (B14–18, A158/B197). But fewer readers are aware that this assertion, whether right or wrong, is certainly the boldest and perhaps also the most important claim in post-Cartesian metaphysics. This is because it posits the thesis of modal dualism, or the claim that there are two irreducibly different basic types of necessary truth, in the face of the almost universally-held counter-thesis of modal monism, or the claim that there is one and only basic type of necessary truth, i.e., analytically or logically necessary truth. Given Kant’s theory of truth, modal dualism also implies the worldly existence of two irreducibly different types of modal facts as truth-makers for analytically and synthetically necessary truths respectively. In short if Kant is right, then there are fundamentally more things in heaven and earth than modal monists are prepared to acknowledge. Moreover Kant holds that all the basic statements of traditional metaphysics are, at least in intention, synthetic a priori judgments (B18). Hence his famous critique of traditional metaphysics in the Transcendental Dialectic is nothing but a deepened and extended investigation of the possibility of synthetic a priori judgments.