 In the final chapter of Convention, Lewis gives his theory of what it is for a community to speak a language (see the section on conventional theories of meaning in the entry on  convention),  i.e., for a community to have adopted one language as their language by convention. Lewis individuates languages largely by the truth conditions they assign to sentences. And his account of truth conditions is given in terms of possible worlds. So the truth condition of an indicative sentence is the set of possible worlds in which it is true. Somewhat more abnormally, Lewis takes the truth condition for an imperative to be the set of possible worlds in which the imperative is obeyed. (The account of language in Convention covers many different moods, but we will focus here on the account of indicatives.)