  But sets are, prima facie, abstract objects. So it looks as if those who take propositions to be sets of possible worlds should count as platonists about propositions. But some people, like Lewis (1986a, 83) and Maddy (1990, 59), believe that sets of spatiotemporally located members are spatiotemporally located where and when their members are, in which case sets of spatiotemporally located members are  concrete.[16]  But since it lacks any members, the empty set is not spatiotemporally located. And since there are necessarily false propositions, that is, propositions that are true in no possible world, it is plausible, on this conception of propositions, to identify these propositions with the empty set. So some propositions (at least one) seem to be abstract objects. And functions also seem to be abstract objects. And the truth values True and False seem to be abstract objects as well. So these accounts of propositions as sets of possible worlds or functions from possible worlds to truth values, if they are to be nominalistic accounts of propositions, require some consistent and plausible nominalistic account of pure sets, functions and truth values as concrete objects.