 This idea is developed more fully in “Elusive Knowledge” (1996b). Lewis argues that S knows that p is true iff S is in a position to rule out all possibilities in which p is false. But when we say S knows that p, we don’t mean to quantify over all possibilities there are, only over the salient possibilities. The big advantage of Lewis’s approach is that it lets him explain the appeal of scepticism. When the sceptic starts talking about fantastic possibilities of error, she makes those possibilities salient. Since we can’t rule them out, when we’re talking to the sceptic we can’t say we know very much. But since those possibilities aren’t usually salient, we are usually correct in our knowledge-ascriptions. So Lewis lets the sceptic win any debate they are in, without conceding that ordinary knowledge-ascriptions are false.