Sean Carroll in Preposterous Universe:
Are numbers real? What does that even mean? You can’t kick a number. But you can talk about numbers in useful ways, and we use numbers to talk about the real world. There’s surely a kind of reality there. On the other hand, Luke Skywalker isn’t a real person, but we talk about him all the time. Maybe we can talk about unreal things in useful ways. Jody Azzouni is one of the leading contemporary advocates of nominalism, the view that abstract objects are not “things,” they are merely labels we use in talking about things. A deeply philosophical issue, but one that has implications for how we think about physics and the laws of nature.