  Lewis defines a language as a function that assigns truth-conditions to sentences. More precisely, and ignoring complications such as vagueness and indexicality, a language \(L\) is a mapping that assigns each sentence \(s\) a set of possible worlds \(L(s)\). A sentence \(s\) is “true in \(L\)” iff the actual world belongs to \(L(s)\). There are infinitely many possible languages. We must explain what it is for a given group of agents to use a given language. In other words, what is the “actual language” relation? Lewis proposes: