In trying to make sense of the way we talk about possibility, David Lewis argued that everything that could possibly exist does exist, in some possible world. But our best scientific theories suggest that there are no real facts about what’s possible, and therefore possible worlds aren’t real, argues John Divers.

David Lewis (in)famously argued that just about everything you can think of really exists. Just as zebras exist, so do unicorns. Just as brains exist so do the immaterial minds that Descartes thought were associated with them. And just as there exists a universe that conforms to special relativity, so there exists a universe that conforms to Newton’s laws. What is driving all this is a pair of thoughts: (a) there are real facts about what is possible and (b) these facts consist in the real existence of things in other possible worlds. My response is that (b) is moot because we have no reason to believe that (a) is true. We have no reason to believe that there are real facts about possibility because: (i) only science could tell us that there are, and (ii) it doesn’t.

Lewis’s conception of what there is may be extraordinary, or even shocking, but it is not arbitrary. For there is a governing theoretical principle here: it is that everything that could possibly exist does exist. Nor is Lewis’s conception irrational. For there is an extended argument in support of it – in sum: genuine modal realism (as he calls his view) makes much better sense of a great deal of what we hold true than does the alternative (common-sense, conservative) conception that correlates existence with what is actual rather than with what is possible. The reader will note that for Lewis’s view to be consistent he must have been thinking that what actually exists is only a part of what exists overall. That is, indeed, what he thought.

Lewis’s conception of what there is may be extraordinary, or even shocking, but it is not arbitrary.