In this chapter, I show that there is at least one crucial, non-short, argument, which does not involve arguments about spatiotemporality, why Kant’s subjectivism about the possibility of knowledge, argued in the Transcendental Deduction, must lead to idealism. This has to do with the fact that given the implications of the discursivity thesis, namely, that the domain of possible determination of objects is characterised by limitation, judgements of experience can never reach the completely determined individual, i.e. the thing in itself (...) or the unlimited real, but only objects as objects of possible experience. As such, it can be shown by reference to a key argument from Kant, that Hegel’s famous criticism that Kant is not licensed, on the basis of his core arguments concerning the original-synthetic unity of apperception, to restrict our knowledge to appearances, is mistaken on purely systematic grounds. More specifically, I argue that idealism follows already from the constraints that the use of the categories, in particular the categories of quality, places on the very conceivability of things in themselves. My claim is that, although it is not only possible but also necessary to think things in themselves, it does not follow that by merely thinking them we have a full grasp of the nature of things in themselves, as some important commentators claim we have. We must therefore distinguish between two kinds of conceiving of things in themselves: conceiving in the standard sense of ‘forming the notion of’, and conceiving in the narrow sense of ‘having a determinate intellectual grasp’. So although we must be able notionally to think things in themselves, as the grounds of their appearances, we cannot even conceive, through pure concepts, of how they are in themselves in any determinate, even if merely intellectual, sense. To put it differently, we cannot have a positive conception of things in themselves (this is in line with Kant’s distinction between noumena in the negative and positive senses; cf. B307–9). For support, I resort to a much overlooked chapter in the Critique, concerning the transcendental Ideal, where Kant discusses what it is for a thing to be a thing in itself proper, namely, something that is thoroughly determined. This concerns the real ontological conditions of things, which are not satisfied by the modal categories alone, namely, their existence conditions. I claim that the chief reason why, given Kant’s view of determinative judgement, we cannot determine a thing in itself is because of two connected reasons: (1) a thing in itself is already fully determined and therefore not further determinable and (2) we cannot possibly determine all of the thing’s possible determinations. In this context, I also discuss the notion of material (not: empirical) synthesis—of which Kant speaks in the chapter on the transcendental Ideal—which must be presupposed as the ground of the formal a priori synthesis that grounds possible experience. This material synthesis, which is an idea of reason that defines a thing as thoroughly determined with regard to all of its possible predicates and has mere regulative status, can by implication not be determined by the forms of the understanding, which synthesise only a limited set of predicates. As a result, given this definition of ‘thing in itself’, any object (appearance) as at best44 a limited set of determinations of the thing can never be numerically identical to the thing in itself as thoroughly determined individual. This undercuts a standard assumption about the identity relation between appearances and things in themselves in many contemporary interpretations of Kant’s transcendental idealism. (shrink)