As most of its readers are aware, the Critique of Pure Reason is primarily concerned not with empirical, but with a priori knowledge. For the most part, the Kant of the ﬁrst Critique tends to assume that experience, and the knowledge that is based on it, is unproblematic. The problem with which he is concerned is that of how we can be capable of substantive knowledge independently of experience. At the same time, however, the notion of experience plays a crucial role in the central arguments of the Critique. For, again as most readers of the Critique know, Kant aims to show how we can have synthetic a priori knowledge by showing that the categories, or pure concepts of the understanding, are conditions of the possibility of experience. This means that, whether or not Kant is concerned with the notion of experience for its own sake, his account of a priori knowledge carries with it at least some commitments regarding the character of experience. If the account of a priori knowledge is to be successful, then experience has to be the kind of thing for which the categories can, in principle, serve as conditions of possibility. More specifically, experience must involve not only the senses, but also thought or understanding, for otherwise the claim that it presupposes a certain speciﬁc set of concepts is simply unintelligible. And indeed at least some parts of the Critique, in particular the so-called subjective deduction in the ﬁrst edition, and the briefer passages which correspond to it in the second edition, seem to be intended to show how this requirement is met. That is, they are concerned not so much with showing that experience is governed by the categories, as with elaborating a view of experience as involving conceptual activity überhaupt.