Every possible universe is just as real as our own
In 1986, philosopher David Lewis proposed a radical idea called modal realism: every possible world actually exists. When you wonder what would have happened if you chose differently today, you are not imagining a fiction—you are describing an event unfolding in a concrete, physical universe disconnected from ours. Lewis argued that treating possible worlds as literal reality provides the simplest, most coherent foundation for logic, probability, and human thought.
The Literal Reality of What Might Have Been
In everyday life, people constantly weigh possibilities. We ask what would have happened if a historical battle had ended differently, if an election had swung the other way, or if we had chosen a different career path. In standard philosophical and mathematical logic, statements about what could happen or what must happen are called modal statements. Throughout the twentieth century, logicians found that the most effective way to formalize these statements was to speak in terms of possible worlds. Saying that something is possible translates mathematically to saying it is true in at least one possible world, while saying something is necessary means it is true in all possible worlds.
For most philosophers, possible worlds were convenient fictions, mathematical tools, or abstract sets of sentences. In his 1986 treatise, On the Plurality of Worlds, David Lewis took a step that shocked the philosophical community. He argued that possible worlds are not metaphors, abstract models, or unactualized concepts. Instead, Lewis maintained that every logically possible world exists in the exact same physical, concrete sense that our own universe exists. The events that could have happened are literally happening right now, just not in our vicinity of space and time.
Causal Borders and Cosmic Isolation
To understand Lewis's modal realism, one must understand how he defined a world. In his framework, a world is a unified spatiotemporal system. Everything that is spatially and temporally related to you belongs to this world. If two entities can interact, exchange energy, or share a measurable spatial or temporal distance, they inhabit the same universe. A universe may be vast, containing billions of galaxies across billions of light-years, but its boundary is defined by that complete web of spatial and temporal relations.
It follows from this definition that different possible worlds are entirely causally and spatiotemporally isolated from one another. There is no spatial direction you could travel to reach another possible world, and no passage of time will bring you into contact with one. They do not exist in some overarching mega-space or shared master clock. This sharply distinguishes Lewis's modal realism from certain models in theoretical physics, such as inflationary cosmologies with distant bubble universes or higher-dimensional brane spaces, where universes occupy an overarching geometric framework. In Lewis's modal realism, worlds are utterly disconnected realms of matter, energy, and spacetime.
Actuality as an Indexical Word
A central puzzle of modal realism is what makes our world special. We naturally assume that our world is real while others are merely potential. Lewis resolved this by offering an indexical analysis of the word actual. In linguistics, an indexical is a term whose meaning depends on the context of its utterance, such as the words I, here, or now. When you say the word now, you do not mean that the present moment has a unique ontological status that permanently freezes reality; you simply mean the temporal point at which you are speaking.
Under Lewis's view, actual functions in precisely the same way. When inhabitants of our universe say this world is actual, we are simply referring to the world in which we happen to reside. An inhabitant of another universe, gazing at their own stars, correctly calls their own universe actual and ours merely a non-actual possibility relative to them. Actuality is therefore not an objective badge of superior existence. It is purely a perspective-dependent label, meaning every possible universe is equally real, populated by flesh-and-blood beings who consider themselves the real ones.
Counterparts Rather Than Clones
Modal realism immediately raises questions about personal identity. If there is a world where you decided to study astrophysics instead of philosophy, does that mean you literally exist in two places at once? Lewis rejected this idea, known as trans-world identity. Because worlds are causally and spatiotemporally isolated, an individual can only be present in one world. You are an inseparable part of this specific universe, made of matter that exists only here.
To handle claims about what you might have done, Lewis developed counterpart theory. When you say, I could have been a musician, the statement is true not because you exist in another universe playing an instrument, but because you have a counterpart in another universe who does. A counterpart is an independent individual who resembles you closely in origin, temperament, and history, but whose circumstances diverged. Counterpart theory allows modal realism to preserve our ordinary ways of speaking about counterfactuals and personal possibilities without requiring contradictory claims about single objects being in multiple universes simultaneously.
The Rejection of Abstract Worlds
Many philosophers who found Lewis's concrete universes absurd attempted to construct safer alternatives known collectively as ersatz modal realism. Ersatz realists claimed that possible worlds do exist, but only as abstract objects—such as comprehensive sets of sentences, maximal consistent propositions, or structural diagrams stored in the actual world. In these views, the possible world where an asteroid missed the dinosaurs is just a giant list of true descriptions, not a physical jungle crawling with living creatures.
Lewis mounted a sustained attack against these abstract substitutes, arguing that they cannot do the philosophical work required of them. He argued that ersatz models are inevitably circular or reliant on unexplained magic. To explain what makes an abstract set of sentences consistent or possible, an ersatzist must either invoke unanalyzed modal terms—thereby failing to explain possibility itself—or rely on mysterious, primitive representational relations between abstract shapes and physical reality. Lewis contended that only concrete realism genuinely explains what makes modal assertions true without circular reasoning.
Theoretical Economy and the Incredulous Stare
Lewis was entirely aware of how outrageous his hypothesis sounded. He noted that the most common reaction from fellow philosophers was what he famously termed the incredulous stare: an expression of sheer disbelief rather than a substantive counterargument. Critics objected on several fronts. Epistemologically, if other worlds are causally cut off from us, how could we ever know anything about them? Ethically, if every conceivable horrific crime and benevolent act is guaranteed to occur in some concrete world, why should our local decisions matter?
Lewis defended his system through a standard of theoretical utility, comparing his move to the introduction of sets in mathematics or unobservable particles in physics. While positing concrete worlds is an immense quantitative cost—it populates the cosmos with infinitely many entities—it is qualitatively parsimonious. It does not invent bizarre new categories of existence; it simply multiplies the familiar kinds of concrete entities we already know exist. For Lewis, the immense clarifying power modal realism brought to logic, semantics, probability theory, and the philosophy of mind was well worth the price of taking possibilities literally.
Key takeaways
•David Lewis proposed that all logically possible worlds exist as concrete, physical realities entirely disconnected from our own spacetime.
•The word 'actual' is indexical, functioning like 'here' or 'now', meaning our universe enjoys no superior ontological status over any other possible universe.
•Instead of identical individuals existing across multiple universes, counterpart theory holds that distinct individuals in other worlds resemble us and satisfy our claims about what might have been.
•Lewis defended this radical ontology on grounds of explanatory utility, arguing that literal worlds resolve fundamental problems in logic and metaphysics that abstract substitutes cannot.