This paper analyzes Kant’s theory of freedom, interpreting it as a peculiar form of compatibilism. Kant maintains that the distinction between appearances and things in themselves is necessary for resolving the antinomy between freedom and causal determinism. Many scholars consider it a distinction between two different ontological realms and interpret Kant’s theory of freedom based on it. However, this paper aims to show that the key point of Kant’s theory can only come to light under the two-aspect interpretation. This paper argues that the concept of freedom is not only essential for Kant’s moral philosophy, but also indispensable for his theoretical philosophy. It is crucial to understand Kant’s concept of spontaneity, which means the freedom of thinking and cognition, in order to appreciate in what sense Kant claims that the concept of freedom constitutes the keystone of the whole system of pure reason.