On the sources of metaphysics

If one wishes to present a body of cognition as science, then one must first be able to determine precisely the differentia it has in common with no other science, and which is therefore its distinguishing feature; otherwise the boundaries of all the sciences run together, and none of them can be dealt with thoroughly according to its own nature.

Whether this distinguishing feature consists in a difference of the object or the source of cognition, or even of the type of cognition, or several if not all of these things together, the idea of the possible science and its territory depends first of all upon it.

First, concerning the sources of metaphysical cognition, it already lies in the concept of metaphysics that they cannot be empirical. The principles of such cognition (which include not only its fundamental propositions or basic principles, but also its fundamental concepts) must therefore never be taken from experience; for the cognition is supposed to be not physical but metaphysical, i.e., lying beyond experience. Therefore it will be based upon neither outer experience, which constitutes the source of physics proper, nor inner, which provides the foundation of empirical psychology. It is therefore cognition a priori, or from pure understanding and pure reason.