Aristotle, Metaphysics

[990b] [1] As for those who posit the Forms as causes,¹ in the first place in their attempt to find the causes of things in our sensible world, they introduced an equal number of other entities—as though a man who wishes to count things should suppose that it would be impossible when they are few, and should attempt to count them when he has added to them. For the Forms are as many as, or not fewer than, the things in search of whose causes these thinkers were led to the Forms; because corresponding to each thing there is a synonymous entity apart from the substances (and in the case of non-substantial things there is a One over the Many²), both in our everyday world and in the realm of eternal entities.³

Again, not one of the arguments by which we⁴ try to prove that the Forms exist demonstrates our point: from some of them no necessary conclusion follows, and from others it follows that there are Forms of things of which we hold that there are no Forms.For according to the arguments from the sciences⁵ there will be Forms of all things of which there are sciences⁶; and according to the "One-over-Many" argument,⁷ of negations too; and according to the argument that "we have some conception of what has perished," of perishable things; because we have a mental picture of these things.⁸ Again, of Plato's more exact arguments some establish Ideas of relations,⁹ which we do not hold to form a separate genus;and others state the "Third Man."¹⁰ And in general the arguments for the Forms do away with things which are more important to us exponents of the Forms than the existence of the Ideas; [20] for they imply that it is not the Dyad that is primary, but Number¹¹; and that the relative is prior to the absolute¹²; and all the other conclusions in respect of which certain persons, by following up the views held about the Ideas, have gone against the principles of the theory.

Again, according to the assumption by which we hold that the Ideas exist, there will be Forms not only of substances but of many other things (since the concept is one not only in the case of substances, but also in the case of all other things; and there are sciences not only of substances but of other things as well; and there are a thousand other similar consequences); but according to logical necessity, and from the views generally held about them, it follows that if the Forms are participated in, then there can only be Ideas of substances. For they are not participated in qua accidents; each Form can only be participated in in so far as it is not predicated of a subject.I mean, e.g., that if anything participates in "absolute Doubleness" it participates also in "eternal," but only accidentally; because it is an accident of Doubleness to be eternal.¹³Thus the Forms must be substance. But the same names denote substance in the sensible as in the Ideal world;