 Population. If the persistence question is about which of the characters introduced at the beginning of a story have survived to become those at the end of it, we may also ask how many are on the stage at any one time. What determines how many of us there are now? If there are some seven billion people on the earth at present, what facts—biological, psychological, or what have you—make that the right number? The question is not what causes there to be a certain number of people at a given time, but what there being that number consists in. It’s like asking what sort of configuration of pieces amounts to winning a game of chess, rather than what sorts of moves typically lead to winning.