On the Plurality of Worlds

David Lewis is one of the most influential philosophers of our age, and On the Plurality of Worlds is his magnum opus. OPW1 offers an extended development and defense of the hypothesis that there are many universes, things of the same kind as the universe in which we all live, move, and have our being. Lewis calls these universes ―worlds‖, deliberately recalling the notion of a ―possible world ‖ familiar from modal logic and the metaphysics of modality. The title invokes the thesis of the book: there are pluralities of worlds, things of the same kind as the world we inhabit, differing only with respect to what goes on in them. Lewis sought in earlier work (Lewis, 1973, pp. 84–86) to offer a direct argument from common sense modal commitments to the existence of a plurality of worlds.2 OPW offers a less direct argument. Here, Lewis supports the hypothesis by arguing that, if we accept it, we have the material to offer a wide range of analyses of hitherto puzzling and problematic notions. We thereby effect a theoretical unification and simplification: with a small stock of primitives, we can analyze a number of important philosophical notions with a broad range of applications. But the analyses Lewis proposes are adequate only if we accept the thesis that there are a plurality of worlds. Lewis claims that this is a reason to accept the thesis. In his words, «the hypothesis is serviceable, and that is a reason to think that it is true » (p. 3). Thanks are due to Roberto Ciuni for comments on an earlier draft, and for Terence Cuneo and Mark Moyer for discussion.