- The notion of a possible world is familiar from Leibniz’s philosophy, especially the idea – parodied by Voltaire in Candide – that the world we inhabit, the actual world, is the best of all possible worlds. But it was primarily in the latter half of the twentieth century that possible worlds became a mainstay of philosophical theorizing. In areas as diverse as philosophy of language, philosophy of science, epistemology, logic, ethics, and, of course, metaphysics itself, philosophers helped themselves to possible worlds in order to provide analyses of key concepts from their respective domains. David Lewis contributed analyses in all of these fields, most famously, perhaps, his possible worlds analysis of counterfactual conditionals (Lewis 1973). But these analyses invoking possible worlds cry out for a foundation: how is all this talk about possible worlds to be construed? Do possible worlds exist? If so, what is their nature?
- David Lewis boldly responded: this talk of possible worlds is the literal truth. Lewis propounded a thesis of modal realism: the world we inhabit – the entire cosmos of which we are a part – is but one of a vast plurality of worlds, or cosmoi, all causally and spatiotemporally isolated from one another. Whatever might have happened in our world does happen in one or more of these merely possible worlds: there are worlds in which donkeys talk and pigs fly, donkeys and pigs no less “real” or “concrete” than actual donkeys and pigs. Moreover, whatever you might have done but didn’t is done in another possible world by a counterpart of you, someone just like you up until shortly before the time in question, but whose life diverges from yours thereafter. According to modal realism, the actual and the merely possible do not differ in their ontological status. They differ only in their relation to us: merely possible worlds are spatiotemporally and causally inaccessible; we can’t get there from here.
- When David Lewis first endorsed modal realism in the late ‘60’s and early ‘70’s, it elicited “incredulous stares” from other philosophers, even from other practitioners of possible worlds analyses. But by the early ‘80’s, a spate of papers had been published in which those incredulous stares were backed by argument, and in which seemingly “more sensible” approaches to possible worlds were presented, approaches, for example, taking possible worlds to be “abstract objects” of some sort. On the Plurality of Worlds is Lewis’s response: an extended elaboration and defense of modal realism. The greatness of this work lies not so much in its power to persuade – Lewis himself did not think the case for modal realism was, or could be, decisive – but in the masterful presentation of positions and arguments in the metaphysics of modality, and in the many problems in outlying areas of metaphysics that are clarified along the way. It is systematic philosophy at its finest.
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