According to Kant the unity of a person is grounded on the unity of reason. Reason refers to the comprehensive idea of an intelligible world. Knowledge a priori about the world, however, is restricted to the sphere of sensible objects in space and time (nature). Therefore the idea of human freedom, being a presupposition of the autonomy of practical reason, can neither be proved nor be disproved. Since the reality of freedom is inexplicable the categorical imperative demands that we behave as if we were free, respectively, as if we were part of an intelligible world. The faculty of judgment now links theoretical and practical reason by the claim that the purposive unity of the empirical diversity of nature, especially the beauty, presupposes the idea of its intelligible ground. With a special focus on the third Critique the paper aims to clarify Kant's justification of the unity of reason.