Aristotle has just finished discussing the two kinds of errors that problems have. He continues his explanation of how to argue by showing that sometimes something else is mistaken for an accident. He does this in Topics, Book 2.
Now one commonplace rule is to look and see if a man has ascribed as an accident what belongs in some other way. This mistake is most commonly made in regard to the genera of things, e.g. if one were to say that being a colour is an accident of white for being a colour does not happen by accident to white, but colour is its genus. The assertor may of course make the mistake in so many words, saying (e.g.) that it is an accident of justice to be a virtue- but often even without such explicitness it is obvious that he has rendered the genus as an accident- e.g. suppose that one were to say that whiteness is coloured or that walking is in motion. For a predicate drawn from the genus is never ascribed to the species in a derived form, but always the genera are predicated of their species synonymously- for the species take on both the name and the account of their genera. A man therefore who says that white is coloured has not rendered it as its genus, seeing that he has used a derived form, nor yet as its property or as its definition- for the definition and property of a thing belong to it and to nothing else, whereas many things besides white are coloured, e.g. a log, a stone, a man, a horse. Clearly then he renders it as an accident.
One way to check for a false statement is by making sure that we do not mistake something else for an accident. This mistake most commonly happens by confusing a genus with an accident. For example, we may think that being a color is an accident of whiteness. But that is not true. Color is the genus of white. It is possible for the mistake to be this obvious, but it is not always that obvious. One might claim that whiteness is colored or that walking is change. These sorts of statements are describing color as an accident of whiteness and change as an accident of walking. When we speak of one thing belonging to a genus, we say that it is that kind of thing. So we do not say that people have animalness, but that people are animals. Whatever a genus is, that is what the species of a genus are also. This means that we can never say that the genus of a thing is predicated of that thing. So someone who says that whiteness is colored is speaking of color as an accident of whiteness. There are many things that are colored logs, stones, people and horses but they are colored accidentally. Whiteness is a color. So we should not speak about it as if it were an accident.
Next, Aristotle describes how to check for a predicate belonging universally or not to something.