The purpose of this paper is to defend the view that Kant has propounded an internalist theory of moral motivation. In particular, I shall argue that Kant’s espousal of internalism is evidenced by his claim that pure reason’s relation to the will is premised on a practical synthetic a priori proposition. What I aim to demonstrate is that Kant treated practical syntheticity as a pivotal concept for his account of what it means to be motivated by principles of pure reason. (...) On my construal of Kant’s motivational theory, the relation between universalizable maxims and the moral interest to act upon them is necessary but non-tautological, since violations of duty are logically possible despite our having a moral reason to act. What prevents the latter argument from collapsing into a quasiexternalist account of moral motivation is that the motivational impact of law-like maxims is ultimately premised on a normative conception of ourselves as free agents. (shrink)