David K. Lewis:
Languages and Language (1975).
Philosophical Papers I. Oxford 1983, p. 163-188:
"Conventions are regularities in action, or in action and belief, which are arbitrary but perpetuate themselves because they serve some sort of common interest."
"More precisely: a regularity in R, in action or in action and belief, is a convention in a population P if and only if, within P, the following six conditions hold. (Or at least they almost hold. A few exceptions to the ''everyone''s can be tolerated.)
(1) Everyone conforms to R.
(2) Everyone believes that the others conform to R.
(3) This belief that the others conform to R gives everyone a good and decisive reason to conform to R himself. [...]
(4) There is a general preference for general conformity to R rather than slightly-less-than-general conformity – in particular, rather than conformity by all but anyone. [...]
(5) R is not the only possible regularity meeting the last two conditions. [...]
(6) Finally, the various facts listed in conditions (1)-(5) are matters of common (or nutual) knowledge: they are known to everyone, it is known to everyone that they are known to everyone, and so on."
"My proposal [...] is that the convention whereby a population P uses a language £ is a convention of truthfulness and trust in £. To be truthful in £ is to act in a certain way: to try never to utter any sentences of £ that are not true in £. Thus it is to avoid uttering any sentence of £ unless one believes it to be true in £. To be trusting in £ is to form beliefs in a certain way: to impute truthfulness in £ to others, and thus to tend to respond to another's utterance of any sentence of £ by coming to believe that the uttered sentence is true in £."
© Norbert Fries, Online Lexikon Linguistik. Berlin 2006 ff.