In this dissertation, I argue for what I call a formalist interpretation of Kant’s doctrine of transcendental apperception. On this interpretation, transcendental apperception is not a representation of the subject, or any aspect of a subject, such as her internal activities or states. Indeed, I maintain that transcendental apperception should not be understood in representational terms at all. Instead, I argue, transcendental apperception must be conceived of as an end-directed activity of a subject that is the formal condition of a particular kind of representation, thought. On the view I develop, Kant identifies transcendental apperception with a spontaneous activity of the subject, which functions as an inner organizational principle through which representations are combined in such a way that they have a particular kind of unity, a unity that Kant calls ‘formal’ or ‘logical’. It is this kind of unity that a representation must have in order to inhere in a subject not just as a sensible representation, but as an intellectual representation, which, for finite beings like us, are thoughts. In developing this account, I draw on Kant’s faculty psychology and the underlying metaphysics of causal powers that he inherits from the rationalist tradition, to show that his conception of the understanding as a spontaneous faculty signals that he conceives of its operations in teleological terms. This teleological dimension of the understanding helps us to articulate an account of transcendental apperception that breaks from the two dominant approaches in the literature, what I will call the epistemic and psychological approaches. On the epistemic reading, transcendental apperception is a kind of self-consciousness that grounds our cognitive agency, and on the psychological reading, transcendental apperception is a mechanism that functions causally to connect representations to produce judgments. The former account, I argue, is flawed insofar as it introduces normative and epistemic considerations into Kant’s account of the understanding that are incompatible with key aspects of his faculty psychology; and the latter is flawed because it fails to situate the understanding within the teleological explanatory framework signaled by Kant’s claim that the understanding is spontaneous.