Abstract
Three major philosophical views on possible worlds offer important tools for understanding modal structure, yet none can accommodate the possibilities that eternal inflation describes. Lewis’s modal realism individuates worlds by spatiotemporal connection, but eternal inflation produces bubble universes that share common origins while hosting entirely different physics. His criterion would count all such bubbles as parts of a single world, since they remain spatiotemporally connected through a shared inflating substrate. Armstrong’s combinatorialism builds possible worlds from recombinations of actual constituents, but different vacuum states involve properties that no actual constituent in our world instantiates. Armstrong explicitly denies the possibility of such “alien universals,” leaving his system without the resources to represent these bubbles as possible worlds. Lange’s counterfactual stability account successfully analyzes laws within fixed spacetimes, but it presupposes a clean separation between laws and initial conditions that becomes untenable when spacetime geometry is itself dynamical, and his three-level hierarchy of necessity leaves no room for the four-level structure that, I argue, an adequate account of the possibilities eternal inflation describes would require.