Kant's characteristic distinction between sensibility and understanding is generally taken to mark the shift from his pre-critical period to the mature critical period. It is at the heart both of his explanation of the possibility of synthetic a priori cognition and of his critique of speculative metaphysics. As we shall see, for Kant, sensibility both extends our cognition, allowing us to go beyond mere concepts to synthetic a priori cognition, and constrains our synthetic a priori cognition to objects of possible experience.

Sensibility in the pre-critical period

Kant introduced a fundamental distinction between the faculties of sensibility and intellect in his Inaugural Dissertation of 1770, De mundi sensibilis atque intelligibilis forma et principiis (On the Form and Principles of the Sensible and Intelligible World), in which he makes clear the importance of this distinction for metaphysics. He describes De mundi as a specimen of a “propaedeutic science” for metaphysics that “teaches the distinction between sensitive cognition and the cognition which derives from the understanding” (ID 2:395). If we keep straight the sources of our concepts, Kant claims, we can avoid certain longstanding metaphysical disputes. One such dispute that had concerned Kant at least as early as 1755 revolved around an apparent conflict between the metaphysical doctrine of indivisible monads and the infinite divisibility of geometrical space. In his Physical Monadology, Kant asked: