Appendix 10.

(500) [Plato's species and genera]

It is certain, however, that in many places Plato spoke hesitatingly about his ideas. It seems that for him ideas were abstract, that is, ideas of things without accidents. But this way of speaking (for which I shall explain the reason later) does not detract from his basic thought. St. Thomas, in explaining Plato's work, uses words which hint at my own interpretation of Plato's ideas. For example, he says that Plato makes the species substances of individual things (in Aristotle's Metaph., bk. 7, less. 16). Individual things considered in their perfection differ, and to each of them corresponds an idea, an exemplar, used by the creator of the thing to mould and form it, and to form and mould new individuals, provided these can be reduced to their unique common type. This interpretation of Plato seems to me to be confirmed by everything Aristotle says (in Metaph., book 8, less. 16) about the way in which Plato arrived at his teaching.

Moreover, a statement of Plato about species and genera gives weight to the sense I attribute to him. He speaks about predicating something common to several entia, but in such a way that it is applicable first to one ens rather than another (secundum prius et posterius). In this case, what is common cannot exist per se and separately from the entia to which it is attributed; and this is proper to genus. But if what is common is predicated equally of several entia, it exists per se outside the beings to which it is attributed; and this is proper to species. According to Plato, therefore, individuals of a species must be perfectly equal, at least in their positive characteristics, without differing in dignity. I conclude that by his species Plato meant only universal, not abstract, ideas; these universal ideas contain all that is perceived in an individual except the reality of the individual, or the matter and, more generally speaking, the subsistence which is never contained in ideas, as I have shown (cf. 401-403).

If Plato's species are understood in this way, it seems to me that some of Aristotle's objections can be dismissed. For example, Aristotle's attempt to prove against Plato that the matter must form the species of things (Metaph. book 8) now appears a simple equivocation, a misunderstanding. When we think a corporeal thing, we certainly think of the matter composing the thing, but our idea of the matter is not the matter itself. If species means idea, matter does not enter into species. St. Thomas, in order to dispel the equivocation and in some way defend Aristotle, said that matter, which formed part of the species, was not the same matter as the proper principle of the individual, but a kind of general matter (that is, the idea of matter): 'Flesh and bones are found in the concept of human being, but not the flesh and bones of the living Socrates and Plato' (Contra Gent., II, q. 92).

In any case, it seems that in other places Aristotle saw that the principle of species is the universalisation of an individual (I have already indicated this) and not the abstraction. Cf. book 8 of the Metaphysics, where he compares species to numbers and says that any unit increased in number immediately changes its species.