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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.

Such, then, is the method of the discovery [of truth]. For we must begin with the knowledge of the questions to be discussed. For often the form of the expression deceives and confuses and disturbs the mind, so that it is not easy to discover to what class the thing is to be referred; as, for example, whether the foetus be an animal. For, having a conception of an animal and a fœtus, we inquire if it be the case that the fœtus is an animal; that is, if the substance which is in the fœtal state possesses the power of motion, and of sensation besides. So that the inquiry is regarding functions and sensations in a substance previously known. Consequently the man who proposes the question is to be first asked, what he calls an animal. Especially is this to be done whenever we find the same term applied to various purposes; and we must examine whether what is signified by the term is