Possibleworld

Possible world is a complete and consistent way the world could have been. In modal logic and philosophy of language, possible worlds are used as the contexts in which truth conditions of statements involving necessity, possibility, or counterfactuals are evaluated. A sentence is necessarily true if it is true in all worlds accessible from the actual world; it is possibly true if true in some accessible world.

Two major strands define how worlds are understood. In Kripkean semantics, possible worlds are abstract, maximally consistent descriptions or structures that function as alternative states of affairs; accessibility relations connect worlds, determining which are relevant for a given domain of discourse, and similarity relations are key in evaluating counterfactuals. In contrast, David Lewis's modal realism treats possible worlds as fully real, concrete worlds equally existing with the actual world; individuals have counterparts in other worlds, and counterfactuals are true when their consequent holds in the closest world where the antecedent is true.

There are also actualist or paraphrastic accounts that treat possible worlds as abstract, linguistic constructs or as sets of propositions, designed to analyze modality without committing to many concrete universes. The debate touches metaphysics, epistemology, and semantics, affecting theories of necessity, freedom, prediction, and decision. Although the notion remains controversial, possible worlds provide a standard toolkit for rigorously expressing and evaluating modal and counterfactual judgments across philosophy and linguistics.