For if a definition is an account signifying the essence of the thing and the predicates contained therein ought also to be the only ones which are predicated of the thing in what it is, and genera and differentiae are predicated in what it is it is obvious that if one were to get an admission that these are the only attributes predicated in what it is, the account containing them will of necessity be a definition; for it is impossible that anything else should be a definition, seeing that there is not anything else predicated of the object in what it is.