If nominalism means that there is no common nature which we can identify in diverse things, Lloyd Gerson points out that it is not clear how this can keep from implying hyper-nominalism, where there is no common nature by which we can identify the same thing at different times.
We’re familiar with nominalism as extreme empiricism, or the denial that any power transcends the sensual. But the sensual is primarily of the here and now, and so nominalism and empiricism will have to be suspicious of any linking together of the past with the now. But this is exactly what learning is, even on the sensual level. We can posit “rules of association” for the various sensations we have, but the rule cannot be reasonable – from what perspective, and in virtue of what, could we call them reasonable? There can be no rational basis to predication or even the principle of identity. And all this after nominalism started off so reasonable! First, the we find ourselves laughing at how ridiculous it is to talk about things subsisting by their subsistence, or being men by their humanity, or being stop signs by their stopsignyness; but then the costs start to mount when Hume points out that we don’t have selves either; and finally we can dissolve even what was left for Hume when we “see” that there is no reason behind comparing a blue we see now with a blue we remember, or even behind comparing a conclusion to a premise.
It is our nominalist tendency that makes us tend to want to replace truth with fact, because though on the premises of nominalism it is not reasonable to compare this thing to that thing (one blue to another, or an apple to the planets) it is nevertheless a fact that we do. In this sense, all facts are brute facts, and they are even exquisitely well named – for it is proper to brutes to simply associate things without having a reason to associate them. A monkey might, for all I know, associate apples with the moon just as he can associate tigers with danger, but in either case the association is a brute fact, and that’s all there is to it. An abstract noun like gravity, for example, never enters his life as a third thing that serves as the rational basis to associate apples and the moon. And why are we talking about gravity anyway, since it is exactly the same thing to say that heavy object falls by gravity as to say that a human being is human by humanity or a stop sign signifies by its stopsignitude?