Substance, in the truest and primary and most definite sense of the word, is that which is neither predicable of a subject nor present in a subject; for instance, the individual man or horse.But in a secondary sense those things are called substances within which, as species, the primary substances are included; also those which, as genera, include the species.For instance, the individual man is included in the species 'man', and the genus to which the species belongs is 'animal'; these, therefore-that is to say, the species 'man' and the genus 'animal,-are termed secondary substances.
It is plain from what has been said that both the name and the definition of the predicate must be predicable of the subject.For instance, 'man' is predicted of the individual man.Now in this case the name of the species man' is applied to the individual, for we use the term 'man' in describing the individual; and the definition of 'man' will also be predicated of the individual man, for the individual man is both man andanimal.Thus, both the name and the definition of the species are predicable of the individual.
With regard, on the other hand, to those things which are present in a subject, it is generally the case that neither their name nor their definition is predicable of that in which they are present.Though, however, the definition is never predicable, there is nothing in certain cases to prevent the name being used.For instance, 'white' being present in a body is predicated of that in which it is present, for a body is called white: the definition, however, of the colour white' is never predicable of the body.
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 is either predicated of primary substances, or is present in them, and if these last did not exist, it would be impossible for anything else to 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 description 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.Nowthe 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 not included within these, on the other.For these are the subjects of all such.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.