Metaphysics
Modal Realism
All possible worlds exist — David Lewis's bold metaphysical thesis
Modal realism is David Lewis's view that all possible worlds genuinely exist as concrete realities. Lewis (1986): when we talk about how things "could be" or "must be," we're talking about possible worlds. These aren't abstract entities or mental representations — they're real physical worlds, just disconnected from ours. Implications: any consistent way reality could be, is the way some world is. World where dinosaurs survived; world where you took different career; world with different physics. Critics: ontologically extravagant; "incredulous stare" objection. Defenders: theoretical benefits justify ontology.
- AuthorDavid Lewis (1986, "On the Plurality of Worlds")
- ThesisAll possible worlds genuinely exist
- WorldsConcrete realities, not abstract entities
- Causal isolationWorlds don't interact with each other
- JustificationOntological cost paid for theoretical benefits
- Major critique"Incredulous stare" — too extravagant
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Why modal realism matters
- Metaphysics. Bold systematic view.
- Modal logic. Possible worlds semantics.
- Counterfactuals. "What if" analysis.
- Philosophy of language. Meaning and modality.
- Multiverse interpretations. Connection to physics.
- Educational. Interesting metaphysical position.
- Theoretical methodology. Trade-offs in theory choice.
Common misconceptions
- Same as physics multiverse. Different: Lewis's worlds causally isolated.
- Just thought experiment. Lewis intended literally.
- Most philosophers agree. Most don't; Lewis is famous outlier.
- Worlds connect to ours. Causally isolated.
- Just for modal logic. Lewis aimed at broader theory.
- Easy to refute. Lewis defended at length.
Frequently asked questions
What's modal realism?
David Lewis (1986). Possible worlds are real, concrete objects. When we say "X could happen," there's a world where X happens. Not abstract; not mental; concrete. World where I'm a doctor; world where dinosaurs survived; world where physics differs. All exist. We're in actual world (just one of many). Other worlds: causally isolated from ours.
What problems does it solve?
Many. (1) Counterfactuals: "if X had happened, Y would have" — interpretable as: "in worlds closest to ours where X happens, Y happens." (2) Modality: necessity = true in all worlds; possibility = true in some world. (3) Properties: similar/different across worlds. (4) Mathematical structures: concrete implementation. Lewis: theoretical advantages compensate for ontological extravagance.
What's the "incredulous stare"?
Common reaction. "You're saying ALL these worlds exist? Even crazy ones?" Lewis's response: yes; bite the bullet. Theoretical benefits justify. Critics: too extravagant; should prefer ontologically simpler theories. Lewis: theoretical economy not the only criterion. Ontological cost vs theoretical benefit. Most philosophers: don't accept modal realism but appreciate Lewis's reasoning.
What's actual world special about?
Indexical view. "Actual" is like "here" or "now." Each world is "actual" from its own perspective. Just as "here" picks out wherever you are, "actual" picks out whatever world you're in. No world is privileged. Surprising claim — most people think our world is special. Lewis: just our parochial perspective.
How are alternatives different?
(1) Ersatzism: possible worlds are abstract entities (sets of propositions), not concrete. Most contemporary modal theorists. (2) Combinatorialism: worlds are recombinations of properties. Different ontology. (3) Fictionalism: possible-world talk is useful fiction. (4) Realism without concrete pluralism. Lewis: concrete realism extreme but consistent. Others: more parsimonious alternatives.
What about modal logic?
Modal logic (Kripke 1959+): formalizes necessity, possibility. Uses "possible worlds" as semantic devices. Most use ersatzist (abstract) interpretation. Lewis's modal realism: takes possible worlds metaphysically seriously. Different question: does formal apparatus require concrete worlds, or can it work with abstract entities? Most: abstract suffices.
What's the theoretical benefit?
Reductive analysis of modality. Possibility/necessity = quantification over worlds. No need for primitive modal notions. Counterfactuals analyzed as similarity relations between worlds. Useful in: philosophy of language, logic, metaphysics. But: at cost of believing in many concrete worlds. Trade-off: theoretical economy vs ontological economy.