If the word nature is taken simply in its formal meaning, where it means the first inner principle of all that belongs to the existence of a thing, then there can be as many different natural sciences as there are specifically different things, each of which must contain its own peculiar inner principle of the determinations belonging to its existence. But nature is also taken otherwise in its material meaning, not as a constitution, but as the sum total of all things, insofar as they can be objects of our senses, and thus also of experience. Nature, in this meaning, is therefore understood as the whole of all appearances, that is, the sensible world, excluding all nonsensible objects. Now nature, taken in this meaning of the word, has two principal parts, in accordance with the principal division of our senses, where the one contains the objects of the outer senses, the other the object of inner sense. In this meaning, therefore, a twofold doctrine of nature is possible, the doctrine of body and the doctrine of the soul, where the first considers extended nature, the second thinking nature.

Every doctrine that is supposed to be a system, that is, a whole of cognition ordered according to principles, is called a science.