 Lewis claims that we can incorporate both perspectives within a single view. Natural languages, he says, are a proper subset of the set of languages. More specifically, a natural language is a function L from sentences to meanings that a particular population uses, where this use of L is governed by a convention of “truthfulness and trust in L”. To understand this proposal, we need to understand (a) Lewis’s conception of the motive a population might have to use L, (b) what he means by “convention”, and (c) how he thinks a language—a function from sentences to meanings—could figure within a convention thus understood.