David Lewis (philosopher)

David Kellogg Lewis (September 28, 1941 – October 14, 2001) was an American philosopher.

He went on to Swarthmore College and spent a year at Oxford University (1959–60), where he was tutored by Iris Murdoch and attended lectures by Gilbert Ryle, H. P. Grice, P. F. Strawson, and J. L. Austin.

His year at Oxford played an important role in his decision to study philosophy.

Lewis joined the philosophy department at the University of California, Los Angeles, in 1966.

Lewis received his Ph.D. from Harvard University in 1967, where he studied under W. V. O. Quine, whose views he would later dispute.

It was there he took a seminar with the Australian philosopher J. J. C. Smart.

Smart recalled, "I taught David Lewis, or rather, he taught me."

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.

Co-ordination problems are problematic, for, though the participants have common interests, there are several solutions.

Sometimes one of the solutions is "salient", a concept invented by the game-theorist and economist Thomas Schelling (by whom Lewis was much inspired).

For example, a co-ordination problem that has the form of a meeting may have a salient solution if there is only one possible spot to meet in town.

But in most cases, we must rely on what Lewis calls "precedent" for a salient solution.

If both participants know that a particular co-ordination problem, say "which side should we drive on?", has been solved in the same way numerous times before, both know that both know this, both know that both know that both know this, etc. (this particular state Lewis calls common knowledge, and it has since been much discussed by philosophers and game theorists), then they will easily solve the problem.

That they have solved the problem successfully will be seen by even more people, and thus the convention will spread in the society.

A convention is thus a behavioral regularity that sustains itself because it serves the interests of everyone involved.

Another important feature of a convention is that a convention could be entirely different: one could just as well drive on the left; it is more or less arbitrary that one drives on the right in the US, for example.

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.

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.

In 1970, he moved to Princeton University, where he spent the remainder of his career.

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.

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.

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.

Lewis was born in Oberlin, Ohio, to John D. Lewis, a professor of government at Oberlin College, and Ruth Ewart Kellogg Lewis, a medieval historian.

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.

Lewis attended Oberlin High School, where he attended college lectures in chemistry.