 How are synthetic a priori propositions possible? This question is often times understood to frame the investigations at issue in Kant’s Critique of Pure Reason. In answer to it, Kant saw fit to divide the question into three: 1) How are the synthetic a priori propositions of mathematics possible? 2) How are the synthetic a priori propositions of natural science possible? Finally, 3) How are the synthetic a priori propositions of metaphysics possible? In systematic fashion, Kant responds to each of these questions. The answer to question one is broadly found in the Transcendental Aesthetic, and the doctrine of the transcendental ideality of space and time. The answer to question two is found in the Transcendental Analytic, where Kant seeks to demonstrate the essential role played by the categories in grounding the possibility of knowledge and experience. The answer to question three is found in the Transcendental Dialectic, and it is a resoundingly blunt conclusion: the synthetic a priori propositions that characterize metaphysics are not really possible at all. Metaphysics, that is, is inherently dialectical. Kant’s Critique of Pure Reason is thus as well known for what it rejects as for what it defends. Thus, in the Dialectic, Kant turns his attention to the central disciplines of traditional, rationalist, metaphysics — rational psychology, rational cosmology, and rational theology. Kant aims to reveal the errors that plague each of these fields.