David Lewis (philosopher)

Since his death a number of posthumous papers have been published, on topics ranging from truth and causation to philosophy of physics. Lewisian Themes, a collection of papers on his philosophy, was published in 2004. A two-volume collection of his correspondence, Philosophical Letters of David K. Lewis, was published in 2020. A 2015 poll of philosophers conducted by Brian Leiter ranked Lewis the fourth most important Anglophone philosopher active between 1945 and 2000, behind only Quine, Kripke, and Rawls.

Lewis suffered from severe diabetes for much of his life, which eventually grew worse and led to kidney failure. In July 2000 he received a kidney transplant from his wife Stephanie. The transplant allowed him to work and travel for another year, before he died suddenly and unexpectedly from further complications of his diabetes, on October 14, 2001.

Lewis's monograph Parts of Classes (1991), on the foundations of mathematics, sketched a reduction of set theory and Peano arithmetic to mereology and plural quantification. Very soon after its publication, Lewis became dissatisfied with some aspects of its argument; it is currently out of print (his paper "Mathematics is megethology", in Papers in Philosophical Logic, is partly a summary and partly a revision of "Parts of Classes").

Lewis made significant contributions in philosophy of mind, philosophy of probability, epistemology, philosophical logic, aesthetics, philosophy of mathematics, philosophy of time and philosophy of science. In most of these fields he is considered among the most important figures of recent decades. But Lewis is most famous for his work in metaphysics, philosophy of language and semantics, in which his books On the Plurality of Worlds (1986) and Counterfactuals (1973) are considered classics. His works on the logic and semantics of counterfactual conditionals are broadly used by philosophers and linguists along with a competing account from Robert Stalnaker; together the Stalnaker-Lewis theory of counterfactuals has become perhaps the most pervasive and influential account of its type in the philosophical and linguistic literature. His metaphysics incorporated seminal contributions to quantified modal logic, the development of counterpart theory, counterfactual causation, and the position called "Humean supervenience". Most comprehensively in On the Plurality of Worlds, Lewis defended modal realism: the view that possible worlds exist as concrete entities in logical space, and that our world is one among many equally real possible ones.

This theory has faced a number of criticisms. In particular, it is not clear how we could know what goes on in other worlds. After all, they are causally disconnected from ours; we can't look into them to see what is going on there. A related objection is that, while people are concerned with what they could have done, they are not concerned with what people in other worlds, no matter how similar to them, do. As Saul Kripke once put it, a presidential candidate could not care less whether someone else, in another world, wins an election, but does care whether he himself could have won it (Kripke 1980, p. 45).

Lewis's main goal in the book, however, was not simply to provide an account of convention but rather to investigate the "platitude that language is ruled by convention" (Convention, p. 1.) The book's last two chapters (Signalling Systems and Conventions of Language; cf. also "Languages and Language", 1975) make the case that a population's use of a language consists of conventions of truthfulness and trust among its members. Lewis recasts in this framework notions such as truth and analyticity, claiming that they are better understood as relations between sentences and a language rather than as properties of sentences.

Lewis went on to publish Counterfactuals (1973), which gives a modal analysis of the truth conditions of counterfactual conditionals in possible world semantics and the governing logic for such statements. According to Lewis, the counterfactual "If kangaroos had no tails they would topple over" is true if in all worlds most similar to the actual world where the antecedent "if kangaroos had no tails" is true, the consequent that kangaroos in fact topple over is also true. Lewis introduced the now standard "would" conditional operator □→ to capture these conditionals' logic. A sentence of the form A □→ C is true on Lewis's account for the same reasons given above. If there is a world maximally similar to ours where kangaroos lack tails but do not topple over, the counterfactual is false. The notion of similarity plays a crucial role in the analysis of the conditional. Intuitively, given the importance in our world of tails to kangaroos remaining upright, in the most similar worlds to ours where they have no tails they presumably topple over more frequently and so the counterfactual comes out true. This treatment of counterfactuals is closely related to an independently discovered account of conditionals by Robert Stalnaker, and so this kind of analysis is called Stalnaker-Lewis theory. The crucial areas of dispute between Stalnaker's account and Lewis's are whether these conditionals quantify over constant or variable domains (strict analysis vs. variable-domain analysis) and whether the Limit assumption should be included in the accompanying logic. Linguist Angelika Kratzer has developed a competing theory for counterfactual or subjunctive conditionals, "premise semantics", which aims to give a better heuristic for determining the truth of such statements in light of their often vague and context-sensitive meanings. Kratzer's premise semantics does not diverge from Lewis's for counterfactuals but aims to spread the analysis between context and similarity to give more accurate and concrete predictions for counterfactual truth conditions.

Lewis's first monograph was Convention: A Philosophical Study (1969), which is based on his doctoral dissertation and uses concepts of game theory to analyze the nature of social conventions; it won the American Philosophical Association's first Franklin Matchette Prize for the best book published in philosophy by a philosopher under 40. Lewis claimed that social conventions, such as the convention in most states that one drives on the right (not on the left), the convention that the original caller will re-call if a phone conversation is interrupted, etc., are solutions to so-called "'co-ordination problems'". Co-ordination problems were at the time of Lewis's book an under-discussed kind of game-theoretical problem; most game-theoretical discussion had centered on problems where the participants are in conflict, such as the prisoner's dilemma.

Lewis had already proposed this view in some of his earlier papers: "Counterpart Theory and Quantified Modal Logic" (1968), "Anselm and Actuality" (1970), and "Counterparts of Persons and their Bodies" (1971). The theory was widely considered implausible, but Lewis urged that it be taken seriously. Most often the idea that there exist infinitely many causally isolated universes, each as real as our own but different from it in some way, and that alluding to objects in this universe as necessary to explain what makes certain counterfactual statements true but not others, meets with what Lewis calls the "incredulous stare" (Lewis, On the Plurality of Worlds, 2005, pp. 135–137). He defends and elaborates his theory of extreme modal realism, while insisting that there is nothing extreme about it, in On the Plurality of Worlds (1986). Lewis acknowledges that his theory is contrary to common sense, but believes its advantages far outweigh this disadvantage, and that therefore we should not be hesitant to pay this price.

Lewis joined the philosophy department at UCLA in 1966. In 1970, he moved to Princeton, where he spent the remainder of his career.

Lewis was born in Oberlin, Ohio, to John D. Lewis, a Professor of Government at Oberlin College, and Ruth Ewart Kellogg Lewis, a distinguished medieval historian, through whom he was the grandson of the Presbyterian minister Edwin Henry Kellogg and the great-grandson of the Presbyterian missionary and Hindi expert Samuel H. Kellogg. The formidable intellect for which he was known later in his life was already manifest during his years at Oberlin High School, when he attended college lectures in chemistry. He went on to Swarthmore College and spent a year at Oxford University (1959–1960), where he was tutored by Iris Murdoch and attended lectures by Gilbert Ryle, H. P. Grice, P. F. Strawson, and J. L. Austin. It was his year at Oxford that played a seminal role in his decision to study philosophy, and that made him the quintessentially analytic philosopher he soon became. Lewis received his Ph.D from Harvard University in 1967, where he studied under W. V. O. Quine, many of whose views he came to repudiate. It was there that his connection with Australia was first established when he took a seminar with J. J. C. Smart, a leading Australian philosopher. "I taught David Lewis," Smart would say in later years, "Or rather, he taught me."

David Kellogg Lewis (September 28, 1941 – October 14, 2001) was an American philosopher who is widely regarded as one of the most important philosophers of the 20th century. Lewis taught briefly at UCLA and then at Princeton University from 1970 until his death. He is closely associated with Australia, whose philosophical community he visited almost annually for more than 30 years.