Vat.gr.244

...of another, and they are predicated in the question of "what it is," but rather in the question of "what kind of thing it is" or "how it is disposed." For when some ask what that is of which these are predicated, then we answer. But we do not answer the differences and the accident. For they are not predicated in the question of "what it is" of the subject, but rather in the question of "what kind of thing it is." In asking what kind of thing a man is, we say "rational." In asking what kind of thing a raven is, we say "black." "Rational" is a difference, while "black" is an accident. But when we are asked what a man is, we answer "animal."