David Kellogg Lewis (28 Sep 1941 – 14 Oct 2001) was one of the most influential metaphysicians of the 20th and a key figure in the revival of metaphysics in the post-logical empiricist era. He championed the view that quantification over possible worlds involved commitment to the existence of possible worlds, where such worlds exist in the same sense that the actual world exists.
In other respects his philosophy was parsimonious. Much of his output was directed to the project of providing an analysis of claims concerning probability, causation, mind, explanation, counterfactuals, and laws in terms of a doctrine he called ‘Humean supervenience’ – the doctrine that truthmakers for such claims consisted in worlds construed as maximal collections of categorical qualitative properties, closed under spatio-temporal relations, and nothing more. In Lewis’s terms the multiverse of modern cosmology, as defined in eternal inflation or in Everett’s many-worlds interpretation of quantum mechanics, would count as a single world, whereas the multiverse of the string-theory landscape as advocated by Susskind and others would be considered a subset of the set of all possible worlds.
Lewis was born in Oberlin, Ohio, and was educated at Swathmore College and at Oxford University. He received his PhD from Harvard University under W.V. Quine. He taught briefly at UCLA, and from 1970 until his death, at Princeton University.