Thereās a variant of the multiverse theory that says that itās possible that the infinite number of possible universes actually exist. (I donāt know if itās a famous one, but thatās what an atheist I talked to used) Itās the one used to try to refute the intelligence behind the improbable universe we are in. If all possible universes are equally realized, probability would not be an issue (every universe has a 100% chance of existing) and there would seem to be no need for an intelligence to explain what chose which possible universe gets to be realized (although I think intelligence and will is still required to realize each possibility). If thatās the case, since God is a possibility (given his existence has no inherent contradiction), then this premise would necessarily affirm the existence of God in at least one of the universes, and then necessarily in all of the universes.
But multiple universes are entirely extraneous to this sort of argument. Iāve bolded the important premise: that God is possible because his existence is consistent. If that premise is true*, then there is no reason to aver to multiple universes. The nature of possibility is such that, if it is possible that a necessary being exists, then that necessary being exists. It doesnāt have to exist in some concrete universe or other.
Again, the difference between multiple universes and possible worlds is important. There is not an accepted interpretation of possible worlds semantics. Plantinga takes a Platonist interpretation (possible worlds āexistā in an abstract Platonic āthird realmā, while our world is the actual world). David Lewis is famous for his modal realism, in that he thought that each possible world was an actual, concrete universe, and that all possibilities are actual. Most contemporary philosophers, I think, would accept neither of them. A multiple universe hypothesis is not necessarily like either of those. One could take it that way, but just because there are infinite universes doesnāt mean, for instance, that all possibilities are actualized (ie. there doesnāt have to be an alternate universe in which yesterday you joined the circus, although there might be a āpossible worldā in which that occurred).
*This is the question, though. A lot of philosophical arguments are based on the idea that consistent conceivability implies possibility. (For example, arguments for substance dualism based on the separability of the mind and the body.) But generally such a principle needs restrictions.
I think that in many cases our modal intuitions are simply not up to par, and we do not know enough about the āoverflow necessitiesā of our language. A couple examples for the former and the latter:
a). Consider the laws of physics and of nature. We lack a completed physics, so we do not know how they all tie together, if they do, and in what way. We donāt know which of them could have been different (although we might have some ideas, in some places). We donāt know what would account for their being different, either. So can I say that there is a possible world with some different physical constants? I donāt know. Maybe.
b). Rubies are made of aluminum oxide. We have not known this forever. At times, rubies have been confused with other semiprecious gems like topaz. What we have meant by the word ruby was a substance made of aluminum oxide. But we did not have a sufficient theory of chemistry to realize that until recently. That rubies are a variety of aluminum oxide has always been a ānecessityā (in the logical sense) that āoverflowedā our linguistic usage of the term. If someone said, āIn some possible world, rubies are made of quartzā the statement would have been false, although it did not entail a known contradiction.
I think these are good reasons for modal conservatism.
