 Alongside the derivation of his supreme moral principle, the most difficult questions about Kant’s view of practical reason center on its relation to freedom. Although the broad outlines are consistent, Kant’s views on this topic seem to shift more than usual across his critical writings. (See  Kant’s moral philosophy,  §10, for a brief sketch, and Allison 1990 for a masterful, though not uncontroversial, account.) This and the following sub-section focus on Kant’s central, radical claim that “freedom and unconditional practical law [that is, for any finite being, the Categorical Imperative] reciprocally imply each other” (5:29f). On the one hand, freedom implies that practical reason can be pure (non-instrumental, unconditional), and hence that we are subject to the demands of the Categorical Imperative. On the other, our subjection to morality implies that we must be free.