David Kellogg Lewis (September 28, 1941 - October 14, 2001) was an American philosopher. Lewis taught briefly at UCLA and then at Princeton from 1970 until his death. He is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years. He made contributions in philosophy of language, philosophy of mind, philosophy of probability, metaphysics, epistemology, philosophical logic, and aesthetics.
Early work on convention
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 years old. 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 of the game-theoretical discussion had circulated around 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 may be "'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" in order to get 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 behavioural 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 USA, for example.  Lewis's main goal in the book, however, wasn't simply to provide an account of convention but rather to investigate the "platitude that language is ruled by convention" (Convention, p. 1.) The last two chapters of the book (Signalling Systems and Conventions of Language; cf. also "Languages and Language", 1975) make the case that the use of a language in a population consists of conventions of truthfulness and trust among members of the population. Lewis recasts in this framework notions as those of truth and analyticity, claiming that they are better understood as relations between sentences and a language, rather than as properties of sentences.

Q: What is convention?
A: Lewis's first monograph
What year was that written in?
1969
1969