Modal Realism

Definition and Core Thesis

Modal realism is a position in metaphysics about the nature and status of possibility, necessity, and possible worlds. In its most influential form, associated with David Lewis, modal realism maintains that:

• There exist many concrete worlds in addition to the one we inhabit.
• Each of these possible worlds is just as real, concrete, and fully formed as the actual world.
• The term “actual” is indexical: it picks out our world from the standpoint of its inhabitants, in the same way that “here” picks out a location relative to a speaker.

On this view, when we say, “It is possible that things could have been otherwise,” this is understood literally as: there is (in the broadest sense of “is”) some other world in which things are that way. Modal facts—facts about what is possible, necessary, or impossible—are thus grounded in the existence and structure of a plurality of worlds.

Modal realism contrasts with:

• Actualism, which denies that any non-actual entities (such as non-actual worlds or individuals) exist in the same robust sense as actual ones.
• Ersatz modal realism, which accepts talk of “possible worlds” but treats them as abstract representations—for example, sets of propositions or descriptions—rather than as concrete, spatiotemporal realms.

Historical Background and Lewis’s Formulation

Although talk of “possible worlds” appears in the history of philosophy—most notably in Leibniz, who regarded God as surveying all possible worlds before creating the best one—these were not typically taken as literally existing concrete realms. Rather, they were understood as divine ideas, sets of compossible states of affairs, or abstractions.

In the 20th century, modal logic (developed by C. I. Lewis, Rudolf Carnap, Saul Kripke, and others) introduced formal languages for reasoning about necessity and possibility. Kripkean possible-world semantics provided powerful tools for understanding modal statements and intensionality (e.g., belief, knowledge, counterfactuals) by evaluating sentences at different “worlds” or “indices.” Kripke himself did not claim that these worlds are literally real; he treated them more as heuristic or formal devices.

David Lewis radicalized this framework. In works culminating in On the Plurality of Worlds (1986), he argued that the success and utility of possible-world talk were best explained by taking possible worlds at face value. His modal realism has several central components:

• Concrete plurality of worlds: Every possible world is a vast, concrete universe, spatiotemporally unified internally but spatiotemporally and causally isolated from every other world.
• Individuals and counterparts: An individual exists in exactly one world; there are no trans-worldly numerically identical individuals. Instead, we speak of counterparts—distinct individuals in other worlds that resemble a given one in contextually relevant respects.
• Actuality as indexical: “Actual” does not mark an ontological difference but a perspectival one. Every world is actual from the standpoint of its inhabitants.
• Reduction of modality: Claims about what is possible or necessary are analyzed as quantification over worlds (and individuals in those worlds). For example:
  • “Possibly P” = “P is true at some possible world.”
  • “Necessarily P” = “P is true at all possible worlds.”

Lewis’s modal realism is thus both an ontological thesis (about what there is: a vast plurality of worlds) and a reductive program (about how modal truths are grounded).

Arguments For and Against

Arguments in Favor

Proponents, especially Lewis, advance several kinds of arguments:

• Explanatory power and unification: Modal realism offers a unified framework for understanding:

  • Modal claims (possibility, necessity)
  • Counterfactuals (statements about what would have happened if things had been different)
  • Properties, propositions, and similarity relations
    For example, counterfactuals can be analyzed via similarity between worlds: “If A had happened, B would have happened” is true if, in the closest A-worlds, B also obtains.

• Theoretical simplicity (in laws, not ontology): Lewis distinguishes ideological simplicity (few primitive notions) from ontological parsimony (few entities). He argues that modal realism buys significant simplicity in its theory of modality: it reduces modal operators to quantification over worlds using standard logical resources.

• Taking modal discourse literally: Our everyday and scientific practices rely on modal language. Lewis suggests that modal realism respects this language by not reinterpreting it as mere fiction or shorthand; instead, it treats modal claims as straightforwardly about other worlds.

Main Objections

Critics raise several major concerns:

• Ontological extravagance: Perhaps the most famous objection (often summed up as “too many worlds”) is that Lewis’s ontology is unacceptably bloated. To account for every possibility, modal realism posits an immense—typically infinite and extremely varied—array of fully concrete universes. Many philosophers judge this metaphysically costly.

• Epistemological worry: If other worlds are entirely causally isolated, critics ask how we could have any knowledge or justified belief about them. Proponents typically reply that we know modal truths by inference to the best explanation, and the existence of worlds is posited as part of such an explanation, rather than by direct causal contact.

• Intuitiveness and pre-theoretic belief: Many argue that treating merely possible objects and people as being as real as actual ones conflicts with strong intuitive and commonsense views about reality. Lewis acknowledges this clash but maintains that the theoretical benefits outweigh the intuitive costs.

• Counterpart theory objections: Lewis’s reliance on counterpart theory—the idea that we do not exist in multiple worlds but have counterparts there—strikes some as counterintuitive, especially for de re modal claims such as “I could have had different parents.” Critics contend that this seems to attribute possibilities to counterparts, not to the individual in question.

In response to these concerns, some philosophers accept much of Lewis’s framework but search for a more parsimonious ontology, while others reject possible worlds talk altogether or interpret it in strictly formal or fictionalist terms.

Contemporary Variants and Influence

The debate over modal realism has given rise to multiple alternative theories of possible worlds and modality, often defined in opposition to Lewis’s concrete realism:

• Ersatz modal realism (or ersatzism): Possible worlds are treated as abstract entities, such as:

  • Maximally consistent sets of propositions
  • Maximal descriptions or sentences
  • Structured abstract states of affairs
    These views aim to preserve the explanatory roles of worlds while avoiding an infinite array of concrete universes.

• Actualist accounts: Actualists hold that everything that exists, exists in the actual world. Possibility is explained via ways the actual world could have been, described by abstract surrogates rather than by additional realms.

• Modal fictionalism: On this view, discourse about possible worlds is analogous to speaking within a fiction. Statements like “there is a world where…” are taken as true according to a certain useful story, without committing to the literal existence of such worlds.

Despite widespread resistance to its ontology, Lewisian modal realism has had enduring influence:

• It shaped contemporary metaphysics of modality, providing a clear target and framework for alternatives.
• It impacted the semantics of counterfactuals, causation, laws of nature, and properties, since Lewis often appealed to the structure of possible worlds in those domains.
• It sharpened distinctions between different kinds of theoretical virtues (simplicity, parsimony, explanatory scope) and how they trade off in metaphysical theorizing.

In current usage, “modal realism” most often denotes Lewis’s specific, concrete-world view, while “realism about modality” is used more broadly for the idea that there are objective modal facts. The term thus marks one of the most ambitious and controversial proposals in late-20th-century analytic metaphysics.