(a) At the most general level, members of any population often want the others to think and act in a particular way. This requires them to recognize each other’s intentions, share thoughts, and to act accordingly. Members of the population must therefore find a way to display their intentions, etc., and, in equal measure, to recognize the intentions, etc., of others. Using the same language (i.e., the same function from sentences to meanings) to convey their mental states offers an excellent way of meeting this need. It gives members of the population a way to “control others’ beliefs and actions to some extent by means of sounds and marks” (Lewis 1975: 166). Lewis explains how they do this—and in particular how they settle on the same language—by invoking his general notion of a convention.