Prior Analytics VI.4
141a23-141b

Whether, then, a man defines a thing correctly or incorrectly you should examine on these and similar lines. But whether he has mentioned and defined its essence or not, should be examined as follows.

First of all, see if he has failed to make the definition through terms that are prior and more familiar. For a definition is rendered in order to come to know the term stated, and we come to know things by taking not any random terms, but such as are prior and more familiar, as is done in demonstrations (for so it is with all teaching and learning); accordingly, it is clear that a man who does not define things through terms of this kind has not defined at all. Otherwise, there will be more than one definition of the same thing; for clearly he who defines through terms that are prior and more familiar has framed a better definition, so that both will then be definitions of the same object. This sort of thing, however, does not seem to be so; for of each entity there is a single essence; if, then, there are to be a number of definitions of the same thing, the object defined will be the same as the essences represented in each of the definitions; but these are not the same, inasmuch as the definitions are different. Clearly, then, any one who has not defined a thing through terms that are prior and more familiar has not defined it at all.

The statement that a definition has not been made through more familiar terms may be understood in two ways either supposing that its terms are without qualification less intelligible, or supposing that they are less intelligible to us; for either way is possible. Thus the prior without qualification is more familiar than the posterior, a point, for instance, than a line, a line than a plane, and a plane than a solid; just as a unit is more intelligible than a number; for it is prior to and a principle of all number. Likewise, also, a letter is more familiar than a syllable. Whereas to us it sometimes happens that the converse is the case; for a solid falls under perception most of all, and a plane more than a line, and a line more than a point; for most people learn such things earlier; for any ordinary intelligence can grasp them, whereas the others require a precise and exceptional understanding.

Absolutely, then it is better to try to come to know what is posterior through what is prior, inasmuch as such a way of procedure is more scientific. Of course, in dealing with persons who cannot recognize things through terms of that kind, it may perhaps be necessary to frame the account through terms that are familiar to them.