  There are thus several alternative conceptions of abstract objects. But in what follows I shall take abstract objects to be those that are non-spatiotemporal and causally inert. This is because what motivates Nominalism (in one of its senses) is basically the rejection of non-spatiotemporal and causally inert objects. That is, the nominalist sees trouble with abstract objects simply because he sees trouble with non-spatiotemporal, causally inert objects. That this is so can be seen from the fact that nominalist theories are often motivated by empiricist or naturalist views, which find no place for non-spatiotemporal, causally inert   objects.[4]   Thus, for example, one of the main problems with mathematical objects — a subclass of abstract objects — from a nominalist point of view is that it is not easy to see how we can come to have knowledge or form reliable beliefs about them and refer to them, since there are no causal relations between them and us. But this presupposes that what makes abstract objects problematic is their causal inertness. And the source of their causal inertness might be their lack of spatiotemporality.