Does using possible worlds commit you to their existence?

Not necessarily. Concrete modal realists like Lewis affirm robustly existing worlds, but most philosophers adopt abstract or ersatz conceptions on which talk of worlds is a way of describing possibilities without positing other concrete universes.

The Metaphysics of Possible Worlds

Possible worlds are complete ways things could have been. This topic concerns their metaphysical status: whether they are concrete universes, abstract states of affairs, or sets of propositions, and what this implies for the analysis of modality.

Definition

A possible world is a maximal way that reality could be; the metaphysics of possible worlds concerns what such worlds are and whether they exist.

Scope

Covers the ontology of possible worlds rather than their formal semantics: concrete modal realism, ersatz or abstract conceptions of worlds, the analysis of modal claims as quantification over worlds, and the metaphysical costs and benefits of each view.

Core questions

Are possible worlds concrete or abstract?
Do non-actual worlds exist in the same sense as the actual world?
Can modality be reductively analyzed in terms of worlds?
What individuates a world and makes one of them actual?

Key concepts

Possible world
Concrete world
Ersatz world
Actuality
Maximal state of affairs
Reduction of modality

Key theories

Concrete modal realism: Lewis holds that possible worlds are concrete, spatiotemporally isolated universes that exist just as robustly as our own; 'actual' is indexical, picking out the world of the speaker.
Ersatz (abstract) worlds: Worlds are abstract representations, such as maximal consistent sets of propositions or maximal states of affairs; only the actual world is concrete, and other worlds are abstract surrogates.

History

Leibniz spoke of God choosing among possible worlds. After possible worlds became central to modal logic, philosophers debated their nature: Lewis defended concrete modal realism in 1986, while Stalnaker, Plantinga, and Adams developed abstract or ersatz conceptions that avoid Lewis's ontological extravagance.

Debates

Concrete versus ersatz worlds: Lewis argues that only concrete worlds give a genuinely reductive analysis of modality, accepting a vast ontology; ersatzers reply that abstract worlds secure the same theoretical benefits without the incredulous-stare cost.

Key figures

David Lewis
Robert Stalnaker
Alvin Plantinga
Saul Kripke
Gottfried Wilhelm Leibniz

Seminal works

lewis1986
plantinga1974

Frequently asked questions

Does using possible worlds commit you to their existence?: Not necessarily. Concrete modal realists like Lewis affirm robustly existing worlds, but most philosophers adopt abstract or ersatz conceptions on which talk of worlds is a way of describing possibilities without positing other concrete universes.