Island Universes and the Analysis of Modality

Is physical reality a single, unified whole – say, a spacetime of four, or ten, or twenty-six dimensions? Or does physical reality divide into parts that are causally and spatiotemporally isolated from one another, so-called “island universes”? I doubt we could ever have good reason to answer this question one way or the other. In this paper, I focus instead on a prior question, one more amenable to philosophical analysis: Is the supposition that island universes exist even coherent? Is it metaphysically or logically possible? I will argue that island universes are metaphysically possible, and then consider the substantial impact this has upon realist theories of possible worlds and the realist analysis of modality.