 Immanuel Kant explicitly enacts a revolution in epistemology modeled on the Copernican in astronomy. As characteristic of Enlightenment epistemology, Kant, in his Critique of Pure Reason (1781, second edition 1787) undertakes both to determine the limits of our knowledge, and at the same time to provide a foundation of scientific knowledge of nature, and he attempts to do this by examining our human faculties of knowledge critically. Even as he draws strict limits to rational knowledge, he attempts to defend reason as a faculty of knowledge, as playing a necessary role in natural science, in the face of skeptical challenges that reason faces in the period. According to Kant, scientific knowledge of nature is not merely knowledge of what in fact happens in nature, but knowledge of the causal laws of nature according to which what in fact happens must happen. But how is knowledge of necessary causal connection in nature possible? Hume’s investigation of the idea of cause had made clear that we cannot know causal necessity through experience; experience teaches us at most what in fact happens, not what must happen. In addition, Kant’s own earlier critique of principles of rationalism had convinced him that the principles of (“general”) logic also cannot justify knowledge of real necessary connections (in nature); the formal principle of non-contradiction can ground at best the deduction of one proposition from another, but not the claim that one property or event must follow from another in the course of nature. The generalized epistemological problem Kant addresses in the Critique of Pure Reason is: how is science possible (including natural science, mathematics, metaphysics), given that all such knowledge must be (or include) knowledge of real, substantive (not merely logical or formal) necessities. Put in the terms Kant defines, the problem is: how is synthetic, a priori knowledge possible?