The most important element of Kant’s mature metaphysics and epistemology is his doctrine of transcendental idealism, which received its fullest discussion in Critique of Pure Reason (1781/87). Transcendental idealism is the thesis that the empirical world that we experience (the “phenomenal” world of “appearances”) is to be distinguished from the world of things as they are in themselves. The most significant aspect of this distinction is that while the empirical world exists in space and time, things in themselves are neither spatial nor temporal. Transcendental idealism has wide-ranging consequences. On the positive side, Kant takes transcendental idealism to entail an “empirical realism,” according to which humans have direct epistemic access to the natural, physical world and can even have a priori cognition of basic features of all possible experienceable objects. On the negative side, Kant argues that we cannot have knowledge of things in themselves. Further, since traditional metaphysics deals with things in themselves, answers to the questions of traditional metaphysics (for example, regarding God or free will) can never be answered by human minds. This section addresses the development of Kant’s metaphysics and epistemology and then summarizes the most important arguments and conclusions of Kant’s theory.