know what tragelaphos is. Moreover, if he should show what a thing is, and that it is, how will he show this in the same sentence? for both definition and also demonstration manifest one certain thing, but what man is is one thing, and the essence of man is another.
We next say that it is necessary to show by demonstration every thing, that it is, except it be substance, but to be, is not substance to any thing, for being is not the genus. There will then be demonstration that it is, and this the sciences now effect. For what a triangle means, the geometrician assumes, but that it is, he demonstrates. What then will he who defines what it is, prove? that it is a triangle? he then who knows what it is by definition, will not know if it is, but this is impossible.
Evidently then those who define according to the present methods of definition, do not demonstrate that a thing is, for although those lines be equal which are drawn from the middle, yet why is it the thing defined? and why is this a circle? for we might say that there is the same definition of brass. For neither do definitions demonstrate that it is possible for that to be which is asserted, nor that that thing is, of which they say there are definitions, but it is always possible to say why.
If then he who defines shows either what a thing is or what the name signifies, except there is, by no means (an explanation) of what a thing is, definition will be a sentence signifying the same thing as a name, but this is absurd. For in the first place
