What Is a Possible World? Modality, Lewis, and Metaphysics

Direct Answer

A possible world is a complete way things could have been: a full specification of how reality might have been, consistent with the laws of logic. The actual world is one possible world among many — the one that happens to be actual. This idea, drawn from Leibniz ("the best of all possible worlds"), became the backbone of twentieth-century modal logic and metaphysics.

The power of the notion is explanatory. Necessity can be defined as truth in all possible worlds; possibility as truth in some possible world; impossibility as truth in none. Counterfactuals — "if I had not missed the train, I would have arrived on time" — can be analyzed in terms of the nearest possible worlds where the antecedent holds. Whether possible worlds exist, and what they are, has become one of the most important debates in metaphysics.

Historical Context

The idea of possible worlds is usually traced to Leibniz, who conceived God as surveying all possible worlds before choosing the best to create. The modern revival came from logic. In the 1950s and 1960s, Saul Kripke and others built semantics for modal logic using possible worlds: "necessarily P" means P in all worlds; "possibly P" means P in some world. This made modal notions respectable in logic — no longer intuition but structure.

The philosophical question then exploded: what are these worlds? David Lewis's 1986 book On the Plurality of Worlds defended the most radical answer — modal realism: possible worlds are as real as ours, spatiotemporally isolated concrete universes. Others proposed ersatzism: possible worlds are representations — maximal consistent sets of propositions, or models constructed in the actual world. The debate, between realism and actualism, has shaped the metaphysics of modality ever since.

Key Concepts

Possible world. A complete way things could have been; a maximal consistent scenario.

Actualism. The view that only the actual world and its contents exist; possible worlds are abstract representations, not concrete realities.

Modal realism (Lewis). The view that all possible worlds exist concretely; the actual world is just the world we happen to inhabit, and "actual" is an indexical like "here."

Necessity and possibility. Necessity = truth in all possible worlds; possibility = truth in some; the definitions that made modal logic possible.

Counterfactual. A conditional about what would have happened had things been different; analyzed by Lewis via similarity relations among possible worlds.

Philosophical Perspectives

Lewis's modal realism is bold and counterintuitive: there are infinitely many concrete worlds — worlds with talking donkeys, worlds with no universe at all — and they are as real as ours. Its virtues: the analysis is uniform, reductive, and parsimonious in its primitive notions; necessity is just truth in all worlds. Its cost: believing in an infinite plurality of concrete worlds, which many philosophers find excessive.

Actualists reply that we can have the benefits without the ontology. On ersatzism, possible worlds are constructions: sets of propositions, or maximal states of affairs, existing as abstract objects in the actual world. The problem for actualism is to say which representations are "possible" without already using modal notions — the problem of explaining modality without circularity.

The debate matters for science and language. Physics uses counterfactuals constantly ("if the field had been stronger, the particle would have..."); statistics uses possible outcomes; ethics uses "what would happen if everyone did that." Whether possible worlds are real or representational, they are the coin of modern modal reasoning — and the metaphysical debate over their nature continues.

Modern Reflection

Possible worlds remain indispensable in logic, linguistics, computer science (Kripke structures for verification), and the semantics of conditionals in the sciences. The philosophical frontier has moved to "grounding": whether modality is fundamental or derived from the actual structure of things. Hume's old claim — that necessity is a habit of the mind — haunts the field: do we discover necessary truths, or do we impose them?

The ethics and metaphysics of possibility also return in applied form: the "counterfactual history" debates, the modal dimension of free will ("could have done otherwise"), and the multiverse speculation in cosmology, which makes Lewis's plurality look almost tame. Whether one is a modal realist, an actualist, or a skeptic about modality, the possible world is the concept through which modern philosophy thinks about what is necessary, what is possible, and what is not.

Related Thinkers

• Bertrand Russell — logic and the analysis of propositions
• Hume — the skeptical ancestry: necessity as habit
• Leibniz — the best of all possible worlds (context)
• Kant — necessity and the categories

Related Quotes

• "The actual world is one world among many; 'actual' is an indexical." — David Lewis, On the Plurality of Worlds (paraphrase)
• "Necessity is truth in all possible worlds; possibility is truth in some." — modal logic, standard gloss
• "The connection of ideas is not a property of things but a habit of the mind." — Hume, An Enquiry Concerning Human Understanding

Sources

• Possible Worlds — Stanford Encyclopedia of Philosophy
• Modal Logic — Stanford Encyclopedia of Philosophy
• Metaphysics — Stanford Encyclopedia of Philosophy
• On the Plurality of Worlds — David Lewis, Blackwell

Further Learning

• Continue with What Is Metaphysics? and What Is Reality?
• Explore the Understanding Reality collection
• Read about metaphysics and the philosophy of language