If they're not treated seriously, with due care and attention, then there is a danger that one won't be using the notion of possible worlds at all, but some other, ill-defined notion. Transworld Temporality Many philosophers have been guilty of talking as if possible worlds were temporally related to each other. Of course, not all references to times at worlds involve this error. The possible-worlds enthusiast clicks the tongue in mild annoyance and passes on. In order to defend the reality of time against McTaggart and his modern followers, Bigelow appeals to temporal relations between possible worlds.
Capitals represent present events, boldface past events, and italics future events.
Thus, at the first world, event l is in the future, at the second world it is in the present, while at the third world it is in the past. Here we see explicit talk of worlds and their contents existing in each other's past and future. When I say, for example, that England might win the Ashes next time, I'm surely talking about a possible event in the future, so if I'm to capture that in possible-worlds talk, I shall have to talk about worlds at which England win the Ashes in the future . I have something of a problem with this, in so far as I have no idea how to make sense of it. One is given, let's say, a previous history of the world up to a certain time, and from that time it diverges considerably from the actual course.