If, on the other hand, it is not predicated convertibly of the thing, it either is or is not one of the terms contained in the definition of the subject; and if it is one of those terms, then it will be the genus or the differentia, inasmuch as the definition consists of genus and differentiae; whereas, if it is not one of those terms, clearly it would be an accident, for accident was said to be what belongs to a subject without being either its definition or its genus or a property.