Heracleides of Pontus said that every star was a world, a cosmos, with its own earth and its own air. Whatever he meant, we at least can see that this, the idea that the visible stars are planetary systems, makes a useful fourth distinction. And each of the meanings carries a different meaning for pluralism. Pluralism in regard to the Earth is trivial. A pluralism of stars, each potentially accompanied by a planetary system, beyond the limits of what we can see, is what Epicurus might have had in mind. The same within the visible system was perhaps Heracleides' idea. It is difficult to write on the history of atomism without deploring the medieval blurring of the distinctions between Aristotelian, Platonic and Democritean physics. No survey of the cosmological repercussions of atomism would be complete without mention of the notion that we live in a world that is only one of many.