 It is clear that practical reason is the foundation of Kant’s moral philosophy. It is less clear what role reason plays in his theoretical philosophy. Kant insists that “metaphysics is utterly impossible, or at best a disorderly and bungling endeavor” if we do not separate “ideas of reason” from “concepts of the understanding” (Prolegomena §41, 4:329). But while he emphasizes the solidity of empirical knowledge gained through the understanding, reason and its “ideas” often appear as mere sources of error and illusion. (This is especially so for the most-read sections of the first Critique—the Aesthetic, Analytic and Dialectic.) But if this were all that Kant meant to say, the status of philosophical reasoning would stand in grave doubt. In addition, we might note that Kant rarely discusses reason as such. This leaves a difficult interpretative task: just what is Kant’s general and positive account of reason?