In the transcendental argument of the Refutation of Idealism, Kant’s target is not Humean skepticism about the applicability of a priori concepts, but rather Cartesian skepticism about the external world. This argument was added by Kant to the second edition of the Critique of Pure Reason (B274–279, with a change suggested in the Preface to B, Bxxxix–Bxli), and subsequently embellished and reworked in a series of Reflections (Guyer 1987: 279–316). More specifically, Kant intends to refute what h