themselves, although in the whole range of the sensuous world, investigate the nature of its objects as profoundly as we may, we have to do with nothing but phenomena. Thus, we call the rainbow a mere appearance of phenomenon in a sunny shower, and the rain, the reality or thing in itself; and this is right enough, if we understand the latter conception in a merely physical sense, that is, as that which in universal experience, and under whatever conditions of sensuous perception, is known in intuition to be so and so determined, and not otherwise. But if we consider this empirical datum generally, and inquire, without reference to its accordance with all our senses, whether there can be discovered in it aught which represents an object as a thing in itself (the raindrops of course are not such, for they are, as phenomena, empirical objects), the question of the relation of the representation to the object is transcendental; and not only are the raindrops mere phenomena, but even their circular form, nay, the space itself through which they fall, is nothing in itself, but both are mere modifications or fundamental dispositions of our sensuous intuition, whilst the transcendental object remains for us utterly unknown.
The second important concern of our aesthetic is that it does not obtain favour merely as a plausible hypothesis, but possess as undoubted a character of certainty as can be demanded of any theory which is to serve for an organon. In order fully to convince the reader of this certainty, we shall select a case which will serve to make its validity apparent, and also to illustrate what has been said in SS 3.
Suppose, then, that space and time are in themselves objective, and conditions of the—possibility of objects as things in themselves. In the first place, it is evident that both present us, with very many apodeictic and synthetic propositions a priori, but especially space—and for this reason we shall prefer it for investigation at present. As the propositions of geometry are cognized synthetically a priori, and with apodeictic certainty, I inquire: Whence do you obtain propositions of this kind, and on what basis does the understanding rest, in order to arrive at such absolutely necessary and universally valid truths?
There is no other way than through intuitions or conceptions, as such; and these are given either a priori or a posteriori. The latter, namely, empirical conceptions, together with the
