From these twelve categories all the rest may be derived by combination. From the fact that these categories are shown to belong apriori to the understanding, it follows, (1) that these conceptions are apriori, and hence have a necessary and universal validity, (2) that by themselves they are empty forms, and attain a content only through intuitions. But since our intuition is wholly through the sense, these categories have their validity only in their application to the sensuous intuition, which becomes a proper experience only when apprehended in the conceptions of the understanding.—Here we meet a second question; how does this happen? How do objects become subsumed under these forms of the understanding, which for themselves are so empty?
There would be no difficulty with this subsumption if the objects and the conceptions of the understanding were the same in kind. But they are not. Because the objects come to the understanding from the sensory, they are of the nature of the sense. Hence the question arises: how can these sensible objects be subsumed under pure conceptions of the understanding, and fundamental principles (judgments apriori), be formed from them? This cannot result immediately, but there must come in between the two, a third, which must have some thing in common with each, _i. e._ which is in one respect pure and apriori, and in another sensible. The two pure intuitions of the Transcendental Æsthetics, space and time, especially the latter, are of such a nature. A transcendental time determination, as the determination of coetaneousness, corresponds on the one side to the categories, because it is apriori, and on the other side to the phenomenal objects, because every thing phenomenal can be represented only in time. The transcendental time determination, Kant calls in this respect the transcendental _schema_, and the use which the understanding makes of it, he calls the transcendental _schematism_ of the pure understanding. The schema is a product of the imaginative faculty, which self-actively determines the inner sense to this, though the schema is something other than a mere image. An image is always merely an individual and determinate intuition, but the schema merely represents the universal process of the imagination, by which it furnishes for a conception a proper image. Hence the schema can only exist in the conception, and never suffers itself to be brought within the sensuous intuition. If, now, we consider more closely the schematism of the understanding, and seek the transcendental time determination for every category, we find that: