 One thinks, in this [mistaken] picture, of a possible world as if it were like a foreign country. One looks upon it as an observer. Maybe Nixon has moved to the other country and maybe he hasn’t, but one is given only qualities. One can observe all his qualities, but, of course, one doesn’t observe that someone is Nixon. One observes that something has red hair (or green or yellow) but not whether something is Nixon. So we had better have a way of telling in terms of properties when we run into the same thing as we saw before; we had better have a way of telling, when we come across one of these other possible worlds, who was Nixon. (1980, 43)