An issue that has puzzled many philosophers and cosmologists is whether there is just one universe or might there be many. On first inspection the puzzle appears to be bogus. After all, by "universe" is meant "everything that is." Other terms used for this are "existence," "reality," "the world," and so forth. But leave it to the very bright men and women in some of the least accessible disciplines to come up with notions that are very odd, at least to those who like to be grounded on Terra firma in their thinking.
However, without some of these apparent flights of fancy certain valuable discoveries of the past would have been overlooked. So the question is, does the idea of multiple universes qualify as one of these apparent flights of fancy or is it per chance a bona fide and promising flight of fancy?
One way that some people come to believe in multiple universes is by considering whether there is anything contradictory in postulating it. Thus, for example, married bachelors or square circles would not qualify since these are outright self-contradictory. Nothing married can also be a bachelor, nothing that's square can also be a circle. Impossible. So is the idea of multiple universes like these, out and out self-contradictory?
Arguably it is not but then perhaps it is almost. If the meaning of "universe" is "all that exists," then multiple universes would be a self-contradictory idea since if some other thing existed that's like a universe, it would just be part of the universe, not an additional universe. After all, "everything" means just that, everything without exception. But if "universe" means something specifiable, with borders or limits, like a playground or sphere, then it would not be out and out self-contradictory to suppose that there are others beside this one we are familiar with.
Yet, this latter approach relies on changing the meaning of "universe." It no longer is used to mean "everything that exists" but, rather, whatever exists in a certain way and then, quite possibly, other things might exist that way too. Not that there is any reason to think they do only that, there could be no objection to the possibility of their existence. As many cosmologists and philosophers would put it, multiple universes are logically possible – there is no formal contradiction in thinking they exist.
Yet, of course, that alone doesn't establish that multiple universes exist, actually, in reality, as it were. They would be, instead, conceivable, thinkable, perhaps. Like three legged ducks on Mars are – thinkable but with no reason to believe they exist.
But some think that anytime one comes up with some idea that might – just barely might – be realized, it should be treated as in fact existent for in the long run, through eternity, everything possible would in fact be (at some time). Eternity is, after all, a very long time and that gives anything that's even remotely possible some chance of being actual, at some time at least. Who could rule it out – no one could traps around the whole shebang to establish the matter once and for all.
Yet, are we supposed to form our beliefs based on such flimsy possibilities, ones that are deemed possible only because they cannot be ruled out entirely? It seems that the more reasonable thing to do is to regard such logical possibilities – of what might just be possible but no one knows if they are – as mere fancies instead of something worthy of belief. After all, by such reasoning the mere fact that someone might be guilty of a crime of which he or she is accused would justify regarding the person as possibly guilty. This could then lead to their being treated as suspects who should be investigated. Would that be justified? Would it not, instead, amount to harassment?
So the real challenge is what is reasonable to believe in. Many things that do not qualify might possibly be, at some point, somewhere, but unless solid evidence of their reality is at hand, we should most likely postpone any decision about whether they exist. Which, seems to me, goes for multiple universes.