 Given this understanding of what a language is, Lewis goes on to say what it is for a population to speak a language. One natural approach would be to say that speakers and hearers face a co-ordination problem, and settling on one language to communicate in would be a solution to that problem. When Lewis is analysing signalling, that is the approach he takes. But he doesn’t think it will work for language in general. The reason is that he takes conventions to be regularities in action, and it is hard to say in general what actions are taken by hearers.