  The most popular argument for ontological naturalism is epistemological, and consequently ontological naturalism is often allied with epistemological naturalism. If there are abstract entities then it seems we cannot know nor form reliable beliefs about them (Benacerraf 1973, Field 1989), because of their causal isolation from us. Most philosophers take this to be the main problem afflicting platonism. Note that the argument typically leads to agnosticism rather than denial of abstract mathematical objects' existence. This is not the place to engage with the argument—for more details, see Balaguer (2009)—save to sketch how a platonist who is also a scientific or mathematical-cum-scientific naturalist—e.g., Quine—responds to it.