What Are Possible Worlds? (And Does It Matter If They're Real?)
Imagine you’re sitting in your room, thinking about something you did last year that you wish you could take back. You think, “I could have done it differently.” What does that mean, exactly? Not “I should have done it differently” — that’s a different thought. You mean that there was a way things could have gone, different from the way they actually went, and it was possible for you to have taken that path instead.
That seems simple enough. But if you stop and really press on it, it gets weird. What is a “way things could have been”? Where does it exist? Is it just a thought in your head? If so, how can it be something you could have done, when you’re not actually doing it? Or is it — as one philosopher named David Lewis argued — something as real as the chair you’re sitting on?
David Lewis was a philosopher who made big, strange, interesting claims. The strangest one was this: all possible worlds are real. Not just this one. Not just the real one. All of them. There is a world where you chose differently that day. There is a world where dinosaurs never went extinct. There is a world where you were born in a different country, with different parents, and you’re reading a different article right now. And Lewis says these aren’t just fantasies or ideas — they are concrete places, as real as this one.
What Would It Mean for Other Worlds to Be Real?
Let’s start with what Lewis actually believed. He called his view “modal realism.” “Modal” just means having to do with possibility and necessity — words like “could,” “must,” “might,” “necessarily.” So modal realism is the view that possibilities are real.
But what kind of “real”? Not “real as in numbers are real.” Lewis thought numbers are abstract — they don’t exist in space or time. Possible worlds, he said, are concrete. They are made of stuff, the way this world is. They have galaxies and oceans and people (or at least things a lot like people). They just happen to be completely disconnected from us. There is no way to travel to them, no way to communicate with them. They are spatiotemporally isolated from our world. That means they don’t share space or time with us. So you can’t go there, and you can’t see them. But they exist.
Why believe something so strange? Lewis had reasons. The main one was that possible worlds are incredibly useful for explaining all sorts of things.
What Possible Worlds Do for Us
You know how sometimes you think about what would have happened if something had gone differently? Like: “If I had studied harder for that test, I would have gotten a better grade.” Philosophers call these “counterfactuals” — “counter to fact” statements. They’re sentences about what would happen if the world were different in some specific way.
Lewis argued that the best way to understand whether a counterfactual is true is to compare our world to other possible worlds. “If I had studied harder, I would have gotten a better grade” is true if, among the possible worlds closest to ours where you study harder, you get a better grade in most of them.
But here’s the thing: if there aren’t really other worlds — if they’re just ideas or fictions or ways of talking — then it’s hard to say what makes a counterfactual true. Is it just a feeling you have? A rule of language? Lewis thought those answers didn’t work well, and that the simplest, most powerful theory was that there really are other worlds, and that’s what makes counterfactuals true.
Possible worlds also help with explaining what it means to believe something or desire something. Think about a belief: if you believe it’s raining outside, that belief is “true” if the world is a certain way. But what about your other beliefs? Like the belief that you might have been born in a different city? That belief isn’t about this world — there’s no way this world contains you being born elsewhere, because you were actually born where you were. Lewis said that beliefs are really about locating yourself among all possible worlds. To believe you could have been born in Paris is to believe that among all the possible worlds, there are some where your counterpart — your “other self” — was born in Paris.
This way of thinking about beliefs turned out to be very powerful for solving puzzles about self-knowledge. Have you ever had the experience of not knowing who you are? That sounds strange, but think about amnesia stories, or cases where someone gets confusing information about their identity. If you wake up in a hospital with amnesia, you don’t know what world you’re in — you don’t know if you’re in a world where you’re a famous musician or a world where you’re a schoolteacher. Lewis’s theory handles this neatly: to have a belief is to locate yourself among the possible individuals across all worlds.
The Counterpart Problem
Here’s where things get interesting and, for some people, troubling. If there are other worlds with people very much like you, are those people you? Lewis said no.
Think about it this way. You exist in this world. You have a particular body, a particular history, particular parents. Could you have had different parents? Most people think yes — you could have been adopted, or your parents could have met under different circumstances. But could you have been born to completely different parents, in a different century, with a different body? At some point, it stops being you and starts being someone else.
Lewis’s solution was “counterpart theory.” You exist only in this world. You don’t exist in any other world. But you have counterparts in other worlds — individuals who are sufficiently similar to you that they count as the way you might have been. Whether someone in another world counts as your counterpart depends on how similar they are. And here’s the clever bit: similarity comes in degrees, and what counts as “similar enough” can change depending on the context.
So if someone asks, “Could you have been a professional athlete?” we look for worlds where there’s a person very much like you — same background, same talents — who became an athlete. If there are such worlds, and that person is similar enough to be your counterpart, then the answer is yes. If someone asks, “Could you have been born as a fish?” there probably aren’t any worlds where a fish is similar enough to you to count as your counterpart, so the answer is no.
This contextual flexibility is actually a strength, Lewis argued. Our ordinary thinking about what’s possible is flexible in just this way. Sometimes we say, “I could have been a totally different person,” meaning something like: same basic self, different decisions. Other times we say, “I couldn’t possibly have done that,” meaning: that would require me to be a different kind of person entirely. Counterpart theory captures this.
Objections and Replies
Other philosophers had lots of objections to Lewis’s view. Here are a few of the most interesting ones.
Objection: “This is crazy.” Many people’s first reaction to modal realism is that it’s just obviously false. There’s only one world — this one. The others are just ideas. Lewis’s reply was interesting: he didn’t think the objection proved anything. He admitted the view seems strange. But he thought the arguments for it were strong enough to overcome the strangeness. He pointed out that plenty of scientific theories also seem strange at first — quantum mechanics, relativity, the idea that the Earth moves — but we accept them because they work.
Objection: “We can’t know about other worlds.” If other worlds are completely disconnected from ours, how could we possibly know anything about them? Lewis’s reply was that we don’t know about them through observation or experiment. We know about them through reasoning. We know that there must be a world where you chose differently because we know that you could have chosen differently, and the best explanation of that possibility is that there is a world where you did. It’s like knowing about black holes before we could observe them — we knew they must exist because the math said so.
Objection: “This makes morality weird.” If other worlds are real, then there are worlds full of suffering. If you could somehow prevent that suffering, wouldn’t you be obligated to? But you can’t — the worlds are disconnected. Lewis’s reply was that our moral obligations are about what we do, not about what happens in other worlds. The fact that someone in another world is suffering doesn’t mean we’re responsible for it, any more than you’re responsible for a stranger’s suffering on the other side of the world that you couldn’t possibly help.
Why Does Any of This Matter?
You might be wondering: why should a smart 12-year-old care about possible worlds? Here’s one reason: the debate about possible worlds is really a debate about what it means for something to be real.
When you say “unicorns are possible,” what are you saying? Are you saying something about the world, or just about your own imagination? If you’re saying something about the world, then there must be something in the world that makes it true. But what? Lewis’s answer is: there are worlds with unicorns. Other philosophers say: no, what makes it true is that you can imagine unicorns without contradiction. Still others say: what makes it true is that the concept “unicorn” doesn’t break any rules of logic.
What you think about this tells you something about what you think reality includes. Is reality just the stuff you can touch and see? Or does it include possibilities, numbers, meanings, and other invisible things? These are questions that every philosopher has to answer, and they’re questions you can start thinking about right now, with examples from your own life.
The Big Picture
David Lewis died in 2001, but his ideas are still very much alive. Philosophers still argue about whether modal realism is true, whether counterpart theory works, and whether we need possible worlds at all. What most people agree on is that Lewis made the debate sharper and more interesting than it was before.
Here’s a strange thing to think about as you finish reading: if Lewis is right, then somewhere in the vast pluriverse of all possible worlds, there is a version of you who just closed this article