Vat.gr.244

...accident. How then is it predicated in the question of "what it is"? For example, if someone were to ask, "What is a dog?" We answer, "An animal." This is the genus. But if he were to ask again, "What kind of animal is the dog?" We answer, "A barking one." This is the difference. If he were to ask again, "What else has happened to it?" We answer, "Wagging its tail." This is the accident.

One must know that an accident
is never predicated in the question of "what it is,"
but in the question of "what kind of thing it is."
For if we are asked, "What is man?"
we will not say "white," but "animal."
If we are asked "what kind of thing he is,"
we will say "white."

And the properties, and the accidents commonly, but not as if to one. Since there are five, for example, genus, species, difference, property, and accident: genus, for example, animal; species, for example, man; difference, for example, rational; property, for example, risible; accident, for example, black or sitting. Since it is predicated of one thing only, the genus differs in that it is rendered and predicated of many. Since the species is also predicated of many, but not of those differing in species, but in number. For example, man, being a species, is predicated of Socrates and Plato, who do not differ from each other in species, but in number.

But the difference is predicated of many
things that differ in species, such as "rational"
of man, demon, and god.
But not in the question of "what it is,"
but in the question of "what kind of thing it is."
But the genus is in the question of "what it is."

But the property is predicated of many,
but not of those differing in species.
For example, "risible" of Socrates
and Plato.

But the accident is predicated of many,
and of those differing in species, such as
"white" of man, horse, and swan.
But not in the question of "what it is."