They are possible, he said, if it can be shown that human knowledge is dependent upon certain concepts which are not empirical in origin but have their origin in human understanding. However, even before he revealed the existence of such concepts, Kant attempted to show in the first major division of Critique of Pure Reason, entitled the “Transcendental Esthetic,” that a priori considerations form the basis even of human perception or sensibility. This view was important to Kant, for in his proposed Copernican revolution in epistemology, the two sources of knowledge are sensibility and understanding working in inseparable harness together. He had already written in the introduction to Critique of Pure Reason that all knowledge begins with experience, but it does not necessarily arise out of experience.
What are these a priori foundations of sensibility? According to Kant, they are space and time. He reasoned that all objects of perception are necessarily located in space and time. Such objects may vary over a period of time in color, shape, size, and so on and still be perceptible objects, but they cannot be deprived of space and time and still remain perceptible. Even to establish ourselves as perceivers, and objects in our environment as objects of perception, requires the use of spatial and temporal terms—hence, the concepts of space and time. As percipients, we regard perceived objects as separate from or distant from us, and we realize that our perceptions themselves, whether of external objects or of our own thoughts and feelings, succeed one another in time. We cannot represent them otherwise and still sensibly preserve the meaning of the terms “perceiver” and “object of perception.” In this sense, space and time deserve recognition as presuppositions of sense experience. All our empirical, descriptive characterizations of perceptible objects take for granted their fundamental nature as objects in space and time. That is why Kant calls space and time “forms of intuition,” in order to distinguish them from the “contents” of sense experience. To be sure, portions of space and moments of time can be perceived, but such parts must always be understood as forming parts of an underlying continuum of space and time. (British phenomenalists such as Berkeley and Hume were not in agreement with this interpretation of space and time.)