Kant's account of freedom is not merely negative — not simply the absence of external compulsion. A will determined by nothing at all would be random, not free. True freedom is positive: it is the will's capacity to give itself a law through reason. The free will is not uncaused; it is self-caused, governed by its own rational legislation rather than by foreign impulses.
Kant's striking conclusion is that a free will and a will subject to the moral law are the same thing. If the will is truly free — determining itself through reason alone — then the principle it gives itself is precisely the categorical imperative. Morality is not a constraint imposed on freedom; it is what freedom, fully understood, amounts to. To act immorally is, in the deepest sense, to be unfree.
Kant is careful about what philosophy can and cannot establish here. We cannot explain how freedom is possible — this would require knowledge that goes beyond all possible experience. What we can show is that the idea of freedom is not contradictory, and that it is the necessary presupposition of all rational action. The categorical imperative stands; its ultimate metaphysical ground remains, for us, in the dark.
The Third Section of the Groundwork is devoted to this question, culminating in the admission that we cannot comprehend how pure reason can be practical — only that it is.
