David Lewis

David Kellogg Lewis (Oberlin, Ohio, 1941 – Princeton, New Jersey, 2001) was one of the most influential analytic philosophers of the second half of the twentieth century. He is best remembered for his modal realism, but his work also reached into the philosophy of language, logic, the philosophy of mind, epistemology, the philosophy of mathematics and the philosophy of science. He worked within the Anglo-American analytic tradition, with a strong commitment to formal rigour.

The son of a political science professor and a medieval historian, he studied mathematics and chemistry before turning to philosophy after a year at Oxford (1959–60). He earned his doctorate at Harvard in 1967 under Willard Van Orman Quine, whose views he would later challenge. After a brief spell at UCLA he moved to Princeton in 1970 and stayed there until his death. He also maintained close ties with the Australian philosophical community, which he visited almost every year for more than thirty years.

His first major book, Convention (1969), used game theory to examine social conventions: rules such as driving on the right persist because they serve everyone who follows them, and they could have been otherwise. Counterfactuals (1973) followed, offering an analysis of counterfactual conditionals in terms of similarity between possible worlds. Together with the account developed independently by Robert Stalnaker, this theory became a standard reference in logic and linguistics.

His most controversial work, On the Plurality of Worlds (1986), argues that possible worlds are concrete entities, causally isolated from one another and as real as our own. For Lewis, when we say that things would have gone differently in another world, we speak of a real world containing a counterpart of the person in question. The view met strong resistance, which he famously described as the incredulous stare of his critics, and it raises genuine questions about how we could know what happens in worlds causally cut off from ours.

His influence is broad. Few philosophers now accept his version of modal realism, but many contemporary discussions of modality, causation, semantics and metaphysics build on concepts he formulated or reworked.