Everything except primary substances is either predicable of a primary substance or present in a primary substance. This becomes evident by reference to particular instances which occur. "Animal" is predicated of the species "man", therefore of the individual man, for if there were no individual man of whom it could be predicated, it could not be predicated of the species "man" at all. Again, colour is present in body, therefore in individual bodies, for if there were no individual body in which it was present, it could not be present in body at all. Thus everything except primary substances iseither predicated of primary substances, or is present in them, and ifthese last did not exist, it would be impossible for anything elseto exist.
Of secondary substances, the species is more truly substance than the genus, being more nearly related to primary substance. For if any one should render an account of what a primary substance is, he would render a more instructive account, and one more proper to the subject, by stating the species than by stating the genus. Thus, he would give a more instructive account of an individual man by stating that he was man than by stating that he was animal, for the former deion is peculiar to the individual in a greater degree, while the latter is too general. Again, the man who gives an account of the nature of an individual tree will give a more instructive account by mentioning the species "tree" than by mentioning the genus "plant".
Moreover, primary substances are most properly called substances in virtue of the fact that they are the entities which underlie every. else, and that everything else is either predicated of them or present in them. Now the same relation which subsists between primary substance and everything else subsists also between the species and the genus: for the species is to the genus as subject is to predicate, since the genus is predicated of the species, whereas the species cannot be predicated of the genus. Thus we have a second ground for asserting that the species is more truly substance than the genus.
Of species themselves, except in the case of such as are genera, no one is more truly substance than another. We should not give a more appropriate account of the individual man by stating the species to which he belonged, than we should of an individual horse by adopting the same method of definition. In the same way, of primary substances, no one is more truly substance than another; an individual man is not more truly substance than an individual ox.
It is, then, with good reason that of all that remains, when we exclude primary substances, we concede to species and genera alone the name "secondary substance", for these alone of all the predicates convey a knowledge of primary substance. For it is by stating the species or the genus that we appropriately define any individual man; and we shall make our definition more exact by stating the former than by stating the latter. All other things that we state, such as that he is white, that he runs, and so on, are irrelevant to the definition. Thus it is just that these alone, apart from primary substances, should be called substances.
Further, primary substances are most properly so called, because they underlie and are the subjects of everything else. Now the same relation that subsists between primary substance and everything else subsists also between the species and the genus to which the primary substance belongs, on the one hand, and every attribute which is notincluded within these, on the other. For these are the subjects of allsuch. If we call an individual man "skilled in grammar", the predicate is applicable also to the species and to the genus to which he belongs. This law holds good in all cases.
It is a common characteristic of all sub. stance that it is never present in a subject. For primary substance is neither present in a subject nor predicated of a subject; while, with regard to secondary substances, it is clear from the following arguments (apart from others) that they are not present in a subject. For "man" is predicated of the individual man, but is not present in any subject: for manhood is not present in the individual man. In the same way, "animal" is also predicated of the individual man, but is not present in him. Again, when a thing is present in a subject, though the name may quite well be applied to that in which it is present, the definition cannot be applied. Yet of secondary substances, not only the name, but also the definition, applies to the subject: we should use both the definition of the species and that of the genus with reference to the individual man. Thus substance cannot be present in a subject.
Yet this is not peculiar to substance, for it is also the case that differentiae cannot be present in subjects. The characteristics "terrestrial" and "two-footed" are predicated of the species "man", but not present in it. For they are not in man. Moreover, thedefinition of the differentia may be predicated of that of which thedifferentia itself is predicated. For instance, if thecharacteristic "terrestrial" is predicated of the species "man", the definition also of that characteristic may be used to form the predicate of the species "man": for "man" is terrestrial.
The fact that the parts of substances appear to be present in the whole, as in a subject, should not make us apprehensive lest we should have to admit that such parts are not substances: for in explaining the phrase "being present in a subject", we stated" that we meant "otherwise than as parts in a whole".