Possibilism vs. Actualism

The possibilism-actualism debate is a central controversy in the metaphysics of modality concerning the ontological status of possible objects — things that do not actually exist but could have. The core question: Are there merely possible objects (e.g., children that Pope Prevost could have had but did not), or is everything actual? The debate has deep roots in the history of philosophy and is intimately connected to modal-logic, possible worlds semantics, and quantified modal logic. ¹

A Simplifying Introduction

Imagine that a specific person — say, the current Pope — could have had children even though he does not. This seems obviously true as an ordinary-language claim. But what grounds its truth? The possibilist says: there exist merely possible children (possibilia) — objects that don’t actually exist but could have. The actualist denies this, seeking to account for the same modal truths without multiplying ontology beyond what actually exists. The tension is that Kripke-style semantics for modal logic (see kripke-semantics) naturally invites talk of possible worlds and the objects that inhabit them, including merely possible ones — and yet a robust actualism insists that everything that exists is actual. Several sophisticated strategies have been developed to resolve this tension, making the debate one of the most instructive in contemporary metaphysics. ¹

Key Positions

Possibilism (Bi-Modal Conception)

Possibilism holds that there are possibilia — things that are not actual but could have been. Formally:

Poss: ∃x(¬A!x ∧ ◇A!x)

where A! is the actuality predicate. On this view, reality has two modes of being: the robust mode of actuality and the thinner mode of subsistence or mere possibility. This position traces back to Avicenna, the Stoics, and finds modern expression in the work of David Lewis (modal realism — all possible worlds are equally real) and Linsky/Zalta. ¹

Actualism

Actualism is the denial of possibilism: there are no things that contingently fail to be actual. In slogan form: everything that exists is actual. This view faces the challenge of explaining modal truths (e.g., “there could have been something that does not actually exist”) without invoking possibilia. Major actualist strategies include:

1. Haecceitism (Plantinga): Possible worlds are maximal possible states of affairs, not concrete realms. What could have existed are unexemplified haecceities (individual essences) — abstract entities that are themselves actual. See alvin-plantinga. ¹

2. Strict Actualism (Prior): Rejects both possibilia and unexemplified haecceities. Singular propositions supervene on their subjects: had some actual individual a failed to exist, there would be no facts about a. Uses a modified quantified modal logic (system Q) where BF and CBF fail. ¹

3. Perspectivalism: Denies that there is a single absolute notion of “truth at a world.” Instead uses world-stories — consistent sets of sentences — to analyze modality without quantifying over worlds or possibilia. ¹

Necessitism vs. Contingentism (Williamson)

Influential work by Timothy Williamson reframes the debate: what is at issue is not possibilia (since for Williamson everything necessarily exists) but whether there are contingent concrete objects. Necessitism holds that necessarily everything exists necessarily (▫∀x▫∃y y=x). Contingentism denies this. This reframing dissolves the traditional possibilism/actualism distinction, since both Linsky/Zalta (avowed possibilists) and Williamson (necessitist) accept that necessarily everything is actual. ¹

Relation to Quantified Modal Logic

The debate has a precise formal dimension in quantified modal logic. The semantics of quantified modal logic raises two controversial principles:

• Barcan Formula (BF): ◇∃xA → ∃x◇A — if possibly something is A, then there is something that is possibly A.
• Converse Barcan Formula (CBF): ∃x◇A → ◇∃xA — if there is something that is possibly A, then possibly something is A.

Possibilists typically accept both; actualists typically reject them, arguing that they conflate actual quantification with modal possibility. The choice of quantified modal logic (SQML vs. KQML vs. Prior’s Q) corresponds to different metaphysical commitments. ¹

Connection to S5

s5-modal-logic is often the natural logic for possibilism because its equivalence-relation frames (every world sees every other) allow possibilia to be quantified over from any world. Actualists, by contrast, often prefer weaker systems or modified semantics where domains vary across worlds, blocking BF and CBF. The modal reduction principle ◇□A → □A is also central to Leibniz’s ontological argument, which presupposes possibilist-friendly commitments. ¹

Historical Roots

The debate traces back to the Stoics, who distinguished “something” (including mental fictions) from “existents,” and to Avicenna, who held that God creates from possibilities that have their own mode of being. In the early modern period, Leibniz’s possible worlds framework (Leibniz) introduced the notion that necessary truths are true in all possible worlds — but he maintained an actualist metaphysics (only the best world is actualized). The modern debate crystallized with the development of Kripke’s possible worlds semantics and the work of David Lewis (modal realism), Alvin Plantinga (actualist haecceitism), Arthur Prior (strict actualism), and Timothy Williamson (necessitism). ¹

See Also

• modal-logic — The family of modal logics; foundations of necessity and possibility
• s5-modal-logic — The S5 system; equivalence-relation frames where possibilism finds its natural logic
• kripke-semantics — Possible worlds semantics; the formal framework for debating possibilism
• gottfried-wilhelm-leibniz — Possible worlds pioneer; ontological argument in S5
• alvin-plantinga — Haecceitist actualist; possible worlds as maximal states of affairs

References

• Fitch, G. W. & Nelson, M. (2025). “The Possibilism-Actualism Debate.” Stanford Encyclopedia of Philosophy (revised 2025). ¹
• Lewis, D. (1986). On the Plurality of Worlds. Blackwell. — Modal realism.
• Plantinga, A. (1974). The Nature of Necessity. Oxford University Press. — Haecceitist actualism.
• Prior, A. N. (1957). Time and Modality. Oxford University Press. — Strict actualism.
• Williamson, T. (2013). Modal Logic as Metaphysics. Oxford University Press. — Necessitism.

Footnotes

1. raw/articles/sep-possibilism-actualism.md ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹