A complete and maximal way things might have been — a comprehensive specification of entities, properties and relations across a domain — employed as an ontological or representational device to evaluate modal claims (possibility, necessity, counterfactuals).

Principle

Modal claims can be analyzed in terms of existence, truth, or accessibility across possible worlds: a proposition is possible if it is true in at least one such world, necessary if true in all accessible worlds, and counterfactuals are assessed by comparison across nearby worlds.

Demonstration

Situation: We evaluate 'It is possible that there are unicorns.' Recognition: Using a possible‑worlds framework we ask whether there is a complete specification (a possible world) in which creatures fitting 'unicorn' exist. Action: If such a specification exists (or is coherent within the adopted account), we classify the modal claim as true. Consequence: The possible‑worlds device yields systematic evaluations of possibility and necessity, but interpretation depends on whether worlds are treated as concrete, abstract, or merely representational.

Misapplication

Reifying possible worlds as literal concrete spatio‑temporal universes (modal realism) when one intends a representational or heuristic device (ersatzism, modal fictionalism) confuses methodological roles and generates unwarranted ontological commitments.

Consequence

Employing possible worlds provides a uniform semantics for modal discourse and counterfactual reasoning and can clarify entailments and modality, but different ontological readings (concrete realism, ersatzism, conceptualism, actualism) carry distinct metaphysical commitments and costs.

Reversal

Alternative modal theories (actualism, counterpart theory, modal fictionalism, or modal anti‑realism) reject literal maximal worlds or reinterpret them; under these accounts the principle that modal truth reduces to truth in maximal possible worlds must be qualified or replaced.

Boundary

Clearly within: a Leibnizian-style maximal complete specification covering all facts about a way the world might be. Boundary case: partial scenarios or model‑theoretic 'worlds' that fix only relevant variables for a modal judgment. Clearly outside: mere hypotheticals, mental images or non‑maximal descriptions that lack completeness and maximality.

Semantic Tension

Modal Realism ↔ Ersatz/Actualist Accounts — whether possible worlds are concrete entities or representational constructs constrains both semantic analyses and ontological commitment.

Synthesis

'Possible world' is best read as a flexible analytic tool: it offers a powerful uniform semantics for modality but must be interpreted in light of the ontological stance adopted; its usefulness arises from formal coherence, not from any single committed metaphysical picture.