 (c) How could the use of a language in Lewis’s sense (a function pairing sentences with meanings) be a convention in his sense, i.e., a regularity satisfying conditions (1) to (6)? Lewis’s answer is that the regularity, the R, is the practice of being truthful and trusting in a language, L. When this practice is a convention in a community, L is the community’s language. To be truthful in L is to utter a sentence only if one believes that what it means in L holds. To be trusting in L is to believe whatever is meant in L by the sentences one hears uttered. Together, these conventions give rise to a basic two-way cooperation, with speakers being truthful and hearers being trusting. Without this two-way coordination the coordination problem would be difficult (if not impossible) to solve. If speakers are not truthful, what they say is not trustworthy evidence of what they think. If hearers are not trusting, what speakers say is not useful to get hearers to believe and act in the intended manner.