Chapter 10
Since definition is said to be a sentence (explanatory) of what a thing is, it is evident that one definition will be of what a name signifies, or another nominal sentence, as what a thing signifies, which is so far as it is a triangle, which when we know that it is, we inquire why it is. Still it is difficult thus to assume things, the existence of which we do not know, and the cause of this difficulty has been explained before, because neither do we know whether it is or is not, except accidentally. One sentence is indeed in two ways, the one by conjunction, as the Iliad, but the other from signifying one thing of one, not accidentally.
The above-named then is one definition of a definition, but the other definition is a sentence showing why a thing is, so that the former signifies, but does not demonstrate, but the latter will evidently be, as it were, a demonstration of what a thing is, differing from demonstration in the position (of the terms). For there is a difference between saying, why does it thunder? and what is thunder? for thus a person will answer, because fire is extinguished in the clouds; but what is thunder? the sound of fire extinguished in the clouds; hence there is the same sentence spoken in another manner, and in the one way there is a continued demonstration, but in the other there is a de-
