Possible Worlds
When we say something could have been different, are we describing an abstract possibility or a concrete reality that exists somewhere?
Leibniz introduced possible worlds in 1710 to explain divine creation: God chose the actual world from an infinity of alternatives because it was the best one available. Later philosophers, especially David Lewis in 1986, turned the concept into a rigorous framework for analyzing what possibility and necessity even mean.
Lewis, D. (1986). On the Plurality of Worlds. Blackwell.
The basic idea
When you say "I could have been born a year earlier," what are you saying? There's a fact about the actual world: you were born when you were born. The modal claim, that things could have been otherwise, seems to require some account of what that "could" is pointing at.
Possible worlds are a way of making that account precise. A possible world is a complete, consistent way things might have been. The actual world is one such world. Other possible worlds are ones where you were born a year earlier, where Napoleon won at Waterloo, where water is made of something other than H2O. Modal claims, claims about what's possible or necessary, become claims about which possible worlds exist and what's true in them.
Modal realism vs. ersatzism
The interesting dispute is about whether possible worlds are real in any robust sense. David Lewis argued for modal realism: possible worlds exist in exactly the same sense that the actual world exists. They are concrete, spatiotemporally isolated universes. When we say something is possible, we mean it is true in some such universe. When we say something is necessary, we mean it is true in all of them. The actual world is just the one we happen to inhabit.
The rival view, ersatzism, holds that possible worlds are not concrete things but abstract representations: maximal consistent sets of propositions, or descriptions, or structural abstractions. Possible worlds exist, on this view, but only as abstract objects, not as parallel concrete universes.
The debate is not merely academic. Lewis's modal realism is remarkably powerful as a tool for analysis, but the cost of accepting infinitely many concrete universes is high. Ersatzism is more ontologically modest but must explain what it means to say a description is "possible" without circularity.
What the framework actually does
Whatever you think of the metaphysics, possible worlds semantics is a working tool in philosophy. Counterfactuals ("if you had studied harder, you would have passed") can be analyzed as claims about the closest possible world in which the antecedent is true. Statements about necessity ("water is necessarily H2O") mean the statement is true in all possible worlds. Statements about possibility mean it's true in at least one.
The framework also underpins large parts of modal logic, philosophy of language, and philosophy of science. It gives philosophers a rigorous vocabulary for talking about what could have been, what must be, and how things are connected across different ways the world might be.
Discussion questions
- Is possible worlds talk a useful fiction or a description of something real?
- Does it change how you think about your choices to imagine them as occurring in only some of infinitely many possible worlds?
- What does it mean to say something could have happened?
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