And btw, I disagree with orthonorm completely. I don't think that there is a single functional nominalist anywhere. Even the philosophy professors/students who write papers on nominlism do not act like nominalists in real life. When some one refers to a cat, that person does not go through a process of thinking, I see x in front of me, an x resembles y and y resembles z. I know that they are not really the same kind of thing, but for the sake of engaging in thought and speech, I will refer to it with common term "cat."
No one does that!
Here is what all really do on a day to day basis. We see a cat, and we see another cat, and we assume that when we call each a cat, we mean exactly the same thing. Sure we recognize some accidental differences, but we recognize the term cat applying to both in a univocal manner.
People might like to pretend they are nominalists, so that they sound "enlightened" (for some reason, in our day and age, disagreeing with the majority automatically = enlightened), but they don't act like nominalists.
NOTE: This post is not intended to disprove nominalism.