David Lewis Believed Every Possible World Was a Real Place

David Lewis, who taught at Princeton from 1970 until his death in 2001, defended a view his colleagues found difficult to take seriously. The view, set out at length in his 1986 book On the Plurality of Worlds, was that every way the world could have been is a way some world really is. Worlds where the South won the Civil War, worlds with talking donkeys, worlds with different physical constants — all of them concrete, all of them as real as the one you are reading this in.

The technical name is modal realism. Most philosophers had used "possible worlds" as a convenient fiction, a way of cashing out claims like "there could have been more sheep" by saying "in some possible world there are more sheep." Lewis took the talk literally. The other worlds are not abstractions, not stories, not models. They exist. They are causally and spatiotemporally cut off from ours, which is why we can't visit, but they are no less real for that. You exist in this world; in others, your counterparts — separate individuals who resemble you in the relevant respects — exist instead.

The response from his colleagues was largely a stare. Lewis called it the incredulous stare. He acknowledged that the view sounds absurd and argued that absurdity was a poor objection in metaphysics; the right question was whether the view paid for itself. His case was that taking possible worlds literally gave the cleanest account of modality, counterfactuals, properties, and propositions, and that the cleanness was worth the ontological price.

The analogy he leaned on was set theory. Mathematicians postulate enormous infinities of sets to make their theory work, and we don't object that there can't really be that many. Lewis asked metaphysicians to do the same with worlds.

Few were persuaded. The view remains a benchmark — the price-tag any rival account of modality has to beat.