Lewis (1986: 227) offers an argument of this sort, which we can take to involve our imagining a world of one-way eternal recurrence where the history of the actual world repeats itself ad infinitum. Lewis then suggests that we can successfully imagine ourselves occupying different “epochs”—specific recurrences of history from the inside. And, since we can imagine ourselves occupying different epochs, he concludes that some possibilities differ solely with respect to whether one lives in, say, the seventeenth rather than the fortieth epoch. In addition, Lewis (1983, 1986: 239) suggests another conceivability argument that is plausibly taken to require us imagining living the life of one of a pair of twins. (Whether Lewis thinks any robust imaginative undertaking is required here is unclear, but the general features of this particular kind of conceivability argument is our interest here.) And, since we can equally well imagine living the life of the firstborn twin and that of the second-born twin, while holding the qualitative character of the world fixed, it seems that we can successfully imagine possibilities that differ only haecceitistically.