 The basic idea behind the alternative analysis was similar to that proposed by Robert Stalnaker (1968). Let’s say that an A-world is simply a possible world where A is true. Stalnaker had proposed that p □→ q was true just in case the most similar p-world to the actual world is also a q-world. Lewis offered a nice graphic way of thinking about this. He proposed that we think of similarity between worlds as a kind of metric, with the worlds arranged in some large-dimensional space, and more similar worlds being closer to each other than more dissimilar worlds. Then Stalnaker’s idea is that the closest p-world has to be a q-world for p □→ q to be true. Lewis considered several ways of filling out the details of this proposal, three of which will be significant here.