Possible World

Represent a complete way things could be as an alternative point at which propositions can be evaluated.

Version

v1 · 2026-10-31 · History

Domain-specific #

14556

Domain group

Humanities

Origin domain

Philosophy

Subdomains

Modal Logic, Metaphysics → Philosophy

Aliases

Possible alternative world, Modal world

Core Idea

A possible world is a complete alternative way things could be, or a point in a modal interpretation representing that alternative at which propositions can be evaluated. A proposition can be false here but true at another possible world. The usefulness of such points does not establish whether nonactual worlds are concrete realities or abstract representations.^[1]

How would you explain it like I'm…

Another Way Things Could Be

Think about all the different ways everything could have been: maybe you had a cat instead of a dog, or it rained today instead of being sunny. Each complete 'what if' way for everything to be is called a possible world. Thinkers ask in which of these ways a sentence would be true.

Other Ways Things Could Be

A possible world is a complete way things could be — like the world as it is, but with some things different. Philosophers and logicians use possible worlds to ask not just 'Is this true here?' but 'In which possible ways of things being is this true?' Our actual world is just one of them. People argue about what possible worlds really are — whether they're real places, ideas or something else. In logic, a 'world' can also just be a point in a math model where you check whether sentences are true.

Complete Alternative Worlds

A possible world is a complete alternative way things could be, or a point in a formal model that represents such an alternative and where statements can be judged true or false. Instead of only asking whether something is true here, you ask in which alternatives it's true. The actual world is one evaluation point among them; something doesn't have to be actual to be possible. Philosophers debate what these worlds are — concrete realities, abstract states of affairs, sets of propositions or something else. In modal logic, a world is a technical index with its own truth assignments, and a separate relation says which worlds count for words like 'necessarily.' Having a working formal model doesn't settle the philosophical question, and a single world is different from possible-world semantics, the overall framework that uses them.

 

A possible world is a complete alternative way things could be, or a point in a formal interpretation that represents such an alternative and at which statements are evaluated. Possible-world reasoning asks not merely whether a proposition is true here but at which alternatives it is true; the actual world, if designated, is one evaluation point among many, and actuality is not what makes another point possible. The term has two connected but distinct uses. Metaphysically, philosophers dispute what a complete alternative is and whether worlds are concrete realities, abstract states of affairs, maximal sets of propositions or something else. In modal semantics, a world is a model-theoretic index carrying a local valuation or predicate extensions, while a separate accessibility or similarity relation determines which worlds are relevant to a given operator. A workable formal interpretation does not by itself settle the metaphysical ontology. A possible world is also distinct from possible-world semantics, the framework that combines worlds with valuations, relations and truth clauses; the individual alternative and the framework answer different questions.

Scope of Application

Modal logic evaluates necessity and possibility across relevant worlds; epistemic logic can use worlds compatible with an agent's information;^[2] counterfactual semantics can compare antecedent worlds by similarity. Accessibility or similarity selects worlds for a question but is not the identity of one world.^[1][3][4][5]

Clarity

Keep three things separate: an individual world, a relation among worlds, and the full possible-world semantics framework. Also distinguish nonactual from impossible: an impossible world violates the possibility constraints at issue, while a possible world merely need not be actual.^[1]

Manages Complexity

World-indexed truth lets a model store and compare alternative valuations without rewriting an entire hypothetical history for each modal sentence. The finite model still covers only its chosen proposition vocabulary and does not exhaust every fact about a philosophical world.^[1][3]

Abstract Reasoning

Specify a possibility standard and valuation, then evaluate a proposition at each world. Only afterward choose which worlds a modal or epistemic operator inspects. If p is false at the actual point but true at an accessible alternative, it may be possible without being actual.^[1][3]

Knowledge Transfer

The same world-indexed alternative can serve modal, epistemic and counterfactual analyses, though their relations and interpretations differ. Possible-world semantics is a system-level framework, not an alias for one world.

Neighborhood in Abstraction Space

Possible World sits in a sparse region of the domain-specific corpus (68^th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Formal Models & Logical Foundations (37 abstractions)

Nearest neighbors

• Ranking Theory — 0.86
• Impossible World — 0.85
• Logical possibility — 0.83
• Marginal man theory — 0.83
• Formal Model — 0.83

Computed from structural-signature embeddings · 2026-10-31

References

[1] Menzel, Christopher, “Possible Worlds”, Stanford Encyclopedia of Philosophy, introduction and §§1–2; directly inspected philosophical and formal exposition. registry ↩a ↩b ↩c ↩d ↩e

[2] Rendsvig, Rasmus, John Symons, and Yanjing Wang, “Epistemic Logic”, Stanford Encyclopedia of Philosophy, summer 2025 archive, §2.4; directly inspected accessible exposition, not an original 1960s epistemic-logic source. registry ↩

[3] Kripke, Saul A. (1963). “Semantical Considerations on Modal Logic”, Acta Philosophica Fennica 16, 83–94. Original scan discoverable; individual source passages were not directly verified in this review. registry ↩a ↩b ↩c

[4] Starr, Willow (2019). “Counterfactuals”, Stanford Encyclopedia of Philosophy; winter 2025 revision by Starr and Alex Kocurek, §5; directly indexed explanatory account of Lewis–Stalnaker similarity semantics. registry ↩

[5] Lewis, David (1973). Counterfactuals, Harvard University Press. Original scan discoverable; relevant passages were not directly verified in this review. registry ↩