 Two of the most prominent questions in Kant’s critical philosophy concern reason. One question is central to his theoretical philosophy. It arises from the metaphysical assertions of earlier “rationalist” philosophers, especially Leibniz and Descartes. Which claims can reason hope to establish securely? A second question is central to his practical philosophy. It arises from the subservient role accorded to reason by the British empiricists—above all Hume, who declared, “Reason is wholly inactive, and can never be the source of so active a principle as conscience, or a sense of morals” (Treatise, 3.1.1.11; see also the entry on  rationalism vs. empiricism).  What sort of practical relevance can reason claim? These questions are reflected in the titles of two key works: the monumental Critique of Pure Reason, and the Critique of Practical Reason that is middle point of his great trio of moral writings (between the Groundwork of the Metaphysics of Morals and The Metaphysics of Morals).