A metaphysical position that treats possible worlds as concrete, fully real entities on the same ontological footing as the actual world; modal statements (about possibility and necessity) are true or false relative to what occurs in these concrete worlds.

Principle

Modal truth is grounded in the existence of concrete alternate worlds: a proposition is possible if it is true in at least one such world and necessary if it is true in all of them under this ontology.

Demonstration

Illustrative scenario → To evaluate the counterfactual ‘‘If agent A had acted differently, outcome B would have occurred,’’ the modal realist affirms there exists a concrete world where A acts differently and B occurs; the counterfactual’s truth reduces to facts about that world.

Misapplication

Interpreting ‘‘possible world’’ as merely a useful fiction without recognizing that modal realism entails an ontological commitment. The error is failing to track the metaphysical cost of treating possibilities as concrete entities.

Consequence

Endorses a large ontology of non-actual entities with implications for identity across worlds, modal semantics, and metaphysical parsimony—affecting debates about individuation, causation, and counterfactual analysis.

Reversal

Alternative frameworks (e.g., ersatz modal theories, actualist or counterpart accounts) model modality without positing concrete worlds; adopting them avoids the heavy ontological commitment of modal realism while aiming to preserve explanatory power.

Boundary

Clearly within: ontological claims that treat non-actual worlds as concrete objects with full reality. Boundary case: talk of possible scenarios used heuristically in science without ontological commitment. Clearly outside: purely linguistic or epistemic accounts of possibility that deny non-actual concreteness.

Semantic Tension

Ontological parsimony ↔ Explanatory scope: modal realism trades parsimony for a straightforward semantics of modality; rival accounts trade ontology for more economical metaphysics.

Synthesis

Modal realism makes modal discourse ontologically literal: it explains possibility by positing concrete alternatives, clarifying modal semantics at the cost of a sprawling ontology that alternative accounts aim to avoid.