only perceptible when filled. Their perceptibility is Matter, to which I shall return further on, and again in § 21. If Time were the only form of these representations, there could be no coexistence, therefore nothing permanent and no duration. For Time is only perceived when filled, and its course is only perceived by the changes which take place in that which fills it. The permanence of an object is therefore only recognized by contrast with the changes going on in other objects coexistent with it. But the representation of coexistence is impossible in Time alone ; it depends, for its completion, upon the representation of Space ; because, in mere Time, all things follow one another, and in mere Space all things are side by side ; it is accordingly only by the combination of Time and Space that the representation of coexistence arises.
On the other hand, were Space the sole form of this class of representations, there would be no change ; for change or alteration is succession of states, and succession is only possible in Time. We may therefore define Time as the possibility of opposite states in one and the same thing.
Thus we see, that although infinite divisibility and infinite extension are common to both Time and Space, these two forms of empirical representations differ fundamentally, inasmuch as what is essential to the one is without any meaning at all for the other : juxtaposition having no meaning in Time, succession no meaning in Space. The empirical representations which belong to the orderly complex of reality, appear notwithstanding in both forms together ; nay, the intimate union of both is the condition of reality which, in a sense, grows out of them, as a product grows out of its factors. Now it is the Understanding which, by means of its own peculiar function, brings about this union and connects these heterogeneous forms in such a manner, that empirical reality—albeit only for that Understanding—arises out of their mutual interpenetration,