Kant's Reason develops a novel interpretation of Kant's conception of reason and its philosophical significance. Karl Schafer argues that Kant presents a powerful model for understanding the unity of theoretical and practical reason as two manifestations of a unified capacity for theoretical and practical understanding (or "comprehension"). This model allows us to do justice to the deep commonalities between theoretical and practical rationality, without reducing either to the other. In particular, it enables us to see why the activities of both theoretical and practical reason are governed by a version of the Principle of Sufficient Reason, while also seeing why reason is essentially autonomous. At the same time, Kant's Reason reads Kant as presenting a compelling picture of the role that reason, as a capacity or power, should play in a systematic approach to foundational philosophical questions. In doing so, it argues for an account of the fundamental norms that apply to rational beings that treats neither substantive reasons or values nor merely structural rationality as fundamental, but instead provides a robust conception of reason as a power or capacity for theoretical and practical understanding. The result is a form of rational constitutivism, which contrasts both with the forms of reasons fundamentalism that are currently fashionable and the forms of agency-first constitutivism that have dominated Kantian metaethics. In this sense, this volume aims to vindicate Kant's insistence that his philosophy represents nothing more or less than reason's implicit self-understanding coming to explicit and systematic self-consciousness.