The question of the origin of pure rational concepts and the possibility of applying these concepts to the objects of experience is based on the problem of subjective deduction of understanding. Metaphysical deduction of the categories provides an answer to the question about the possibility of pure concepts of understanding. Transcendental deduction explains the constitutive function of categories in the experience. Subjective deduction of understanding establishes the relationship between metaphysical and transcendental deduction of categories and shows the genesis of understanding from the relation of transcendental unity of the apperception “I think” to the transcendental defi nition of time.