Evidently, therefore, with reference to that which is called the genus, none of the species which belong to the genus is either the same as it or other than it in species (rightly so, for the matter is indicated by negation, and the genus is the matter of that of which it is called the genus, not in the sense in which we speak of the genus of the Heraclidae, but in that in which we speak of a genus in nature), nor is it so with reference to things which are not in the same genus, but it will differ in genus from them, and in species from things in the same genus.