Although there are a very few occasions on which Aristotle speaks of words, on the one hand, or mental concepts, on the other, as universals, he was no nominalist and no conceptualist. This negative thesis I have argued sufficiently, at least to my own satisfaction, in an earlier paper. He was, rather, a realist, but of a very tenuous sort. As I said in the earlier paper, he viewed universals as real entities but lacking numerical oneness; each is numerically many, and yet each is also one in some sense. The specific identity of numerically distinct particulars creates something like a class, and this is the universal.
This interpretation was not, in the earlier paper, defended against those who would attribute to Aristotle a much more robust form of realism, and it is that defense which this paper will undertake to provide. The evidence which can be marshalled for the more robust realism is impressive – more so, I would say, than that which can be brought forth for either the nominalist or the conceptualist interpretations. It rests on numerous passages which either explicitly or implicitly seem to tie together as identical, forms, substances, universals, and intelligible objects.