The chapter examines how Kant casts his critical philosophy in the Prolegomena, and in particular how the accusations of being a Berkeleyian force him to refine his views about what exactly the mind-dependence of appearances involves. After looking at what exactly Kant means by ‘a priori intuition,’ the essay explores three main ways to make sense of his transcendental idealism – roughly, an epistemic account, a phenomenalist or ‘mentalist’ view and a relationalist interpretation – and argues that the last of these provides the most fruitful approach to the arguments in the Prolegomena, including the examples involving incongruent counterparts. On this view, the Prolegomena stands as an important stage in Kant’s development precisely in its repudiation of Berkeleyian phenomenalism, and while it is only in the B-edition of the first Critique that transcendental idealism is fully presented, the Prolegomena marks a clear advance over the A-edition.