 Aside from the debate over Quine’s argument, both platonism and nominalism give rise to hard questions. Platonists not only need to provide a theory of what abstract objects exist, but also an account of how we cognitively access and come to know non-causal, abstract entities. This latter question has been the subject of a debate that began in earnest in Benacerraf (1973), which posed just such a dilemma for mathematical objects. Benacerraf noted that the causal theory of reference doesn’t seem to make it possible to know the truth conditions of mathematical statements, and his argument applies to abstract entities more generally. On the other hand, nominalists need to explain the linguistic uses in which we seem to appeal to such entities, especially those uses in what appear to be good explanations, such as those in scientific, mathematical, linguistic, and philosophical pursuits (see Wetzel 2009, 1–22, for a discussion of the many places where abstract types are used in scientific explanations). Even though nominalists argue that there are no abstracta, the very fact that there is disagreement about their existence suggests that both platonists and nominalists acknowledge the distinction between the abstract and concrete to be a meaningful one.