Kant will argue in the Transcendental Aesthetic that human subjects have two pure forms of intuition, space and time, which are the source of synthetic a priori knowledge of mathematics and mechanics. Because these forms are part of the subject's sensibility, he will further conclude that space and time are transcendentally ideal, although they are also empirically real. Thus in the Transcendental Aesthetic Kant takes an important step in establishing the unknowability of things in themselves. These conclusions are based on profound arguments concerning the nature of space and time cognition. Although Kant does not identify his targets, thinkers such as Descartes, Locke, Berkeley, Newton, and Leibniz held opposing theories of both the ontological status of space and time, and our knowledge of them.
THE SENSIBILITY AND THE INTELLECT
Kant says at A22/B36 that the Transcendental Aesthetic will examine the sensibility, to determine whether it contributes a priori knowledge to experience. To accomplish this task, he must isolate the sensibility from the intellect, and then, within sensibility, separate a posteriori from a priori elements. Unfortunately this leaves the impression that Kant's arguments are based on premises concerning the a priori data of sensibility. But his theory of judgment prevents him from proceeding in this manner. In the Transcendental Analytic Kant will argue that all conscious representations, including sense perceptions, must have both sensible and intellectual aspects.