When the materials supplied to our knowledge, having passed through the forms of space and of time, become objects laid out in space and time, then our understanding seizes them. And like our faculty of perception, the faculty of the understanding has its own nature, its own form. Our understanding, to speak like Kant, has its "categories". There are pure a priori concepts of the understanding as there are pure a priori forms of sensibility. Or, to speak more simply, our understanding is an understanding whose essential function is to unify things, to systematize. And it obtains this effect by making the objects it receives from space and time enter into certain frames aimed, precisely, at obtaining this unification. And these frames are the pure a priori concepts of our understanding, at the forefront of which we find the most important of them all, causality. By means of the causal relation, the understanding links up phenomena with other phenomena, objects with objects, and with this relation, and many others, the understanding ends up at an absolutely coherent and systematic unity of nature.