Possible worlds — Complete and consistent ways the world might have been, used to analyze modal notions such as possibility, necessity and contingency. A proposition is possible if true in at least one possible world, necessary if true in all, and contingent if true in some and false in others. The idea has antecedents in Leibniz, who in the Theodicy (1710) holds that God, among infinitely many possible worlds, created the best one, a phrase that became famous as “the best of all possible worlds.” In the twentieth century, possible-worlds semantics acquired formal rigor through Saul Kripke’s work on modal logic in 1959–1963, and was applied to the philosophy of language and metaphysics in Naming and Necessity (lectures of 1970, book of 1980), where Kripke introduces the notion of a rigid designator: a term that designates the same object in every world in which that object exists. The status of the worlds is disputed. For David Lewis (On the Plurality of Worlds, 1986), modal realism holds that other worlds are as real as ours, though not spatiotemporally connected to it. Kripke prefers to treat worlds as stipulated counterfactual situations, not as places to be discovered. Alvin Plantinga (The Nature of Necessity, 1974) defends actualism, according to which worlds are abstract entities existing in the actual world, such as maximal states of affairs. Open questions include transworld identity and the problem of contingency.
See also: Leibniz, Saul Kripke, David Lewis.
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