David Lewis

Lewis's modal realism says that possible worlds are real concrete worlds, not just useful fictions or sets of sentences. He defends this view in On the Plurality of Worlds, published in 1986.

Lewis worked across metaphysics, philosophy of language, mind, decision theory, and counterfactuals. His 1973 book Counterfactuals became especially influential for analyzing statements about what would have happened under different conditions.

Because Lewis treats possible worlds as separate concrete worlds, the same individual does not literally exist in multiple worlds. Instead, modal claims involve counterparts, which are similar individuals in other possible worlds.

The theory is powerful because it gives a clear semantics for necessity, possibility, and counterfactuals. It is controversial because many philosophers reject the idea that countless concrete worlds exist alongside the actual world.