To the extent that modal claims do quantify over possible worlds, the ontological points made about mathematics apply well here too. Since non-actual worlds do not inhabit our spatiotemporal realm, an ontologically naturalist realism seems to be ruled out from the start.  The remaining alternatives are irrealism or non-naturalist realism. The former alternative has been explored in recent years by modal fictionalists (Rosen 1990; Nolan 2011a). The options under the latter heading meet the same epistemological challenges as in the mathematical case: brute intuition faces causal problems; it is contentious whether we should take our best scientific theories to commit us to possible worlds; and, if modal knowledge is to be analytically a priori, on the model of mathematical neo-Fregeanism, then it is not obvious that it can take us to knowledge of possible worlds construed realistically.  