The Aristoteles
The primary mistake of Aristotle, as well as of his followers, was to use the verb "to be" in a single meaning, whereas it actually has two. If it means that a thing is, then individuals alone are, and forms are not; if it means what a thing is, then forms alone are and individuals are not. The controversy on the being of universals has no other origin than the failure of Aristotle himself to make this fundamental distinction. In his philosophy, as much as in that of Plato, what is does not exist, and that which exists is not.
Had Plato lived long enough to read, in the First Book of Aristotle's Metaphysics, the criticism of his own doctrine of ideas, he might have written one more dialogue, the Aristoteles, in which it would have been child's play for Socrates to get Aristotle entangled in hopeless difficulties:
"I should like to know, Aristotle, whether you really mean that there are certain forms of which individual beings partake and from which they derive their names: that men, for instance are men because they partake of the form and essence of man."
"Yes, Socrates, that is what I mean."
"Then each individual partakes of the whole of the essence or else of part of the essence. Can there be any other mode of participation?"
"There cannot be."
"Then do you think that the whole essence is one, and yet being one, is in each one of the things?"
"Why not, Socrates?"
"Because, one and the same thing will then at one and the same time exist as a whole in many separate individuals, and will therefore be in a state of separation from itself!"
"Nay, Socrates, it is not so. Essences are not Ideas; they do not subsist in themselves but only in particular things, and this is why, although we conceive them as one, they can be predicated of many."
"I like your way, Aristotle, of locating one in many places at once; but did you not say that essence is that whereby individual beings are?"
"Yes, Socrates, I did."
"Then, my lad, I wish you could tell me how it may be that beings are through sharing in an essence, which itself is not!"
The history of the problem of universals has precisely been such a dialogue, and it could have no conclusion. If essences exist, they cannot be shared in without losing their unity and consequently their being. If individuals are, then each of them should be a distinct species and there could not be, as in point of fact there are, species that include in their unity a multiplicity of individuals. What is true is that essences are and that individuals exist, so that each essence exists in and through some individual, just as in and through its essence every individual truly is. But, to be in a position to say so, one must first have distinguished between individuation and individuality, that is, one must have realized that, no less necessarily and perhaps more deeply than essence, existence enters the structure of actual being.
