Selection from St. Thomas Aquinas, Commentary on the Metaphysics, trans. John P. Rowan, (Chicago, 1961), bk. 7, lec. 13, nn. 1570-1574.

1570. For the clarification of this chapter it must be noted that the term universal can be taken in two senses. First, it can be taken to mean the nature of the thing to which the intellect attributes the aspect of universality[logical notion], and in this sense universals such as genera and species signify the substances of things inasmuch as they are predicated quidditatively; for animal signifies the substance of the thing of which it is predicated, and so also does man. Second, a universal can be taken insofar as it is universal[metaphysical notion], and insofar as the nature predicated of a thing falls under the aspect of universality, i.e., insofar as animal or man is considered as a one-in-many. And in this sense the Platonists claimed that animal and man in their universal aspect constitute substances.

1571. This is what Aristotle aims to disprove in this chapter[Metaphysics Book 7, chapter 13]by showing that animal in general or man in general is not a substance in reality, but that the form animal or man takes on this generality insofar as it exists in the mind, which understands one form as common to many inasmuch as it abstracts it from all individuating principles. Hence in support of his thesis he gives two arguments.

1572. Concerning the first of these he says that in the light of the succeeding arguments it seems impossible that any one of those attributes which are predicated universally should be a substance, i.e., insofar as it is taken in its universality. This is proved, first, by the fact that while the substance of each thing is proper to each and does not belong to something else, a universal is common to many; for that is said to be universal which belongs by nature to many things and is predicated of many. Hence, if a universal is substance it must be the substance of some thing. Of what thing, then, will it be the substance? For it must either be the substance of all the things to which it belongs or of one. But it is impossible for it to be the substance of all things, because one thing cannot be the substance of many, since those things are many whose substances are many and distinct.

1573. But if it is held to be the substance of one of the things in which it is found, it follows that all other things in which it is found, and of which it is held to be the substance, are that one thing; because it must also be their substance for the same reason, since it is found in all in the same way. Now those things of which the substance and essence are one must also be one themselves. Hence, since a universal cannot be the substance of all the things of which it is predicated or of any one of them, it follows that it is not the substance of anything.

1574. Now it should be noted that he describes a universal as what is naturally disposed to exist in many, and not as what exists in many; because there are some universals which contain under themselves only one singular thing, for example, sun and moon. But this is not to be understood in the sense that the very nature of the species, considered in itself, is not naturally disposed to exist in many things; but there is something else which prevents this, as the fact that all the matter of the species is included in one individual, and the fact that it is not necessary that a species which can last forever in a single individual should be numerically many.