For every question is solved from pre-existing knowledge. And the knowledge pre-existing of each object of investigation is sometimes merely of the essence, while its functions are unknown (as of stones, and plants, and animals, of whose operations we are ignorant), or [the knowledge] of the properties, or powers, or (so to speak) of the qualities inherent in the objects. And sometimes we may know some one or more of those powers or properties,—as, for example, the desires and affections of the soul,—and be ignorant of the essence, and make it the object of investigation. But in many instances, our understanding having assumed all these, the question is, in which of the essences do they thus inhere; for it is after forming conceptions of both—that is, both of essence and operation—in our mind, that we proceed to the question. And there are also some objects, whose operations, along with their essences, we know, but are ignorant of their modifications.