This article considers an apparent Achilles heel for Kant’s transcendental idealism, concerning his account of how synthetic a priori knowledge is possible. The problem is that while Kant’s distinctive attempt to explain synthetic a priori knowledge lies at the heart of his transcendental idealism, this explanation appears to face a dilemma: either the explanation generates a problematic regress, or the explanation it offers gives us no reason to favour transcendental idealism over transcendental realism. In the article, I consider G. E. (...) Moore’s version of the problem, which I argue has not yet received an adequate response. Instead, I offer a way out of this dilemma by focusing on the normativity rather than the metaphysics of the mind. (shrink)