On the Plurality of Worlds
Nathan Salmon1988, The Journal of Philosophy
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Abstract
An Empire of Thin Air (1988) David Lewis's book, On the Plurality of Worlds, is derived from his 1984 John Locke Lectures, and is the latest word on possible worlds from the discipline's foremost champion of possibilia. It is a serious defense, a priori, of Lewis's notorious doctrines (here called 'modal realism') to the effect that there are tiny purple anthropologists who study human culture unobserved, colossal human-eating monsters 50 feet in height, professional philosophers earning annual salaries in excess of 37 million dollars (pre-inflation), and the like, and that these oddities reside in fabulous alternative universes that are never empirically detected by us (but that are empirically detectable by us). The central idea of Lewis's theory is that whatever might have transpired involving individuals of our universe does indeed transpire in one of these alternative universes, involving counterparts of these individuals (p. 2)-'the principle of plenitude.' Equally critical to Lewis's project is the converse principle that everything that transpires in one of these alternative universes involving our counterparts is something that might have transpired involving ourselves. We may call this 'the principle of moderation.' 1 Together these two principles assert an isomorphism between total ways things might have been with regard to this universe and extant alternative universes, prompting Lewis to identify the former with the latter (p. 86). Lewis thus misleadingly calls his alleged alternative universes 'possible worlds,' and indeed they play a role in Lewis's theory of modal discourse similar in many respects to that of the intensional possible worlds invoked in contemporary philosophical semantics, as conceived of by such writers as Saul Kripke and Robert Stalnaker, that is, maximally specific states or histories. 2 These genuine possible worlds Lewis misleadingly labels 'ersatz worlds.' Like genuine possible worlds, Lewis's alternative universes allegedly 'represent' possible events and states of affairs that might have occurred concerning the individuals of our universe; they are supposed to be entities according to which 1 These principles are not explicitly articulated as I have them. My statement of plenitude is based on a plausible interpretation of Lewis's less explicit formulation. Lewis provides a version of moderation which is closely related to, but much weaker than, the principle formulated here, and which he derives from the trivial modal logical truth that whatever is the case might have been the case (p. 5). This weaker principle, however, is insufficient for Lewis's purposes.
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