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 fœtus 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 disputed, or admitted by all. For were one to say that he calls whatever grows and is fed an animal, we shall have again to ask further, whether he considered plants to be animals; and then, after declaring himself to this effect, he must show what it is which is in the fœtal state, and is nourished.