unavoidable difficulties in his undertaking, so that he may not afterwards complain of the obscurity in which the subject itself is deeply involved, or become too soon impatient of the obstacles in his path;—because we have a choice of only two things—either at once to give up all pretensions to knowledge beyond the limits of possible experience, or to bring this critical investigation to completion.
We have been able, with very little trouble, to make it comprehensible how the conceptions of space and time, although à priori cognitions, must necessarily apply to external objects, and render a synthetical cognition of these possible, independently of all experience. For inasmuch as only by means of such pure form of sensibility an object can appear to us, that is, be an object of empirical intuition, space and time are pure intuitions, which contain a priori the condition of the possibility of objects as phenomena, and an à priori synthesis in these intuitions possesses objective validity.
On the other hand, the categories of the understanding do not represent the conditions under which objects are given to us in intuition; objects can consequently appear to us without necessarily connecting themselves with these, and consequently without any necessity binding on the understanding to contain à priori the conditions of these objects. Thus we find ourselves involved in a difficulty which did not present itself in the sphere of sensibility, that is to say, we cannot discover how the subjective conditions of thought can have objective validity, in other words, can become conditions of the possibility of all cognition of objects; —for phenomena may certainly be given to us in intuition without any help from the functions of the understanding. Let us take, for example, the conception of cause, which indicates a peculiar kind of synthesis, namely, that with something, A, something entirely different, B, is connected according to a law. It is not a priori manifest why phenomena should contain anything of this kind (we are of course debarred from appealing for proof to experience, for the objective validity of this conception must be demonstrated à priori), and it hence remains doubtful à priori, whether such a conception be not quite void and without any corresponding object among phenomena. For that objects of
