We have seen above the vast difference between analytic and synthetic judgments. The possibility of analytic propositions could be comprehended very easily; for it is founded solely upon the principle of contradiction. The possibility of synthetic propositions a posteriori, i.e., of such as are drawn from experience, also requires no special explanation; for experience itself is nothing other than a continual conjoining (synthesis) of perceptions. There remain for us therefore only synthetic propositions a priori, whose possibility must be sought or investigated, since it must rest on principles other than the principle of contradiction.

Here, however, we do not need first to seek the possibility of such propositions, i.e., to ask whether they are possible. For there are plenty of them actually given, and indeed with indisputable certainty, and since the method we are now following is to be analytic, we will consequently start from the position: that such synthetic but pure rational cognition is actual; but we must nonetheless next investigate the ground of this possibility, and ask: how this cognition is possible, so that we put ourselves in a position to determine, from the principles of its possibility, the conditions of its use and the extent and boundaries of the same.