This should be relatively short. I just thought I would demonstrate why most philosophers today have given up on Leibniz’ idea that there is such a thing as a best of all (logically) possible worlds. To say that there is such a thing is to say that there is some way we might imagine the world to have been such that no better world could be imagined. This has often been used in the academic literature on Theodicy since Leibniz’ time. The idea goes that if there is such a world, then doubtless God would have actualized it were it in his power. Had he not, he must be something short of omni-benevolent. Had he not the power, perhaps he is something short of omnipotent. Setting aside those two classic objections for the moment to be dealt with another day, it seems to me that before this argument can even get off the ground (at least in these terms) it must be asked whether it is even coherent.
Stephen T. Davis points out in Encountering Evil, Live options in Theodicy:
“In the first place, it is not clear that the notion of the best of all possible worlds is coherent. Take the notion of the tallest conceivable human. This notion is incoherent because, no matter how tall we conceive a tall human to be, we can always conceptually add another inch and thus prove that this person was not, after all, the tallest conceivable human. Just so, it may be argued, the notion of the best of all possible worlds is incoherent. For any possible world, no matter how much pleasure and happiness it contains, we can always think of a better one, i.e., a world with slightly more pleasure and happiness. ” (p.75)
Plantinga illustrates this point candidly when he says:
“Just as there is no greatest prime number, so perhaps there is no best of all possible worlds. Perhaps for any world you mention, replete with dancing girls and deliriously happy sentient creatures, there is an even better world, containing even more dancing girls and deliriously happy sentient creatures. If so, it seems reasonable to think that the second possible world is better than the first. But then it follows that for any possible world W there is a better world W’ , in which case there just isn’t any such thing as the best of all possible worlds.” (God, Freedom, and Evil. P.61)
Now, the only thing which gives me pause has to do with the constraints on our imaginitive capacities, and constraints on what is logically possible. What if actual infinities are logically possible? I have good reasons to think they aren’t and that to posit an actually infinite set leads to logical contradiction. However, just provisionally, let us imagine that infinite sets are not incoherent. That would mean that should we have a world in which there is an actually infinite set of dancing girls and deliriously happy sentient creatures, that would seem to be the best of all possible worlds. We might imagine, then, that something like that is logically possible, and thus we might ask why God did not actualize it. Even if one appeals to a libertarian free-will theodicy this problem will not be entirely avoided: God would have at least actualized a world in which the best of all possible worlds could possibly have been instantiated, but that implies that an actually infinite set of things would have to exist in this world.
Perhaps the best of all possible worlds means a world in which all possible goods are attained. However, another consideration might be that some goods are of the type that they are only possible given pain or suffering. For instance, what would the good of curing somebody of an ailment look like in a world where no such ailment existed? What would repentance and reform look like without sin? These goods could only be purchased by the occasion of a corresponding evil. However, if we follow this argument through, we might end up with something like David Lewis’s extreme modal realism, where every possible way things might have been is a way which things actually are in some other real parallel world. This would imply that God must have created all logically possible worlds in order for there to have actually been all logically possible goods. However, this in itself becomes a theodicy, as every logically possible world must exist, and therefore ours must exist, and therefore we cannot use our world as evidence against God creating the best of all possible worlds (or the best of all possible multi-worlds).
Thomas Aquinas, to my knowledge, takes issue with God creating multiple universes and suggests that God could only have created one single universe. If God created two, then it would need be that the second added to the perfection lacking in the first in some significant way, or else that the second was created gratuitously. However, if it was created because the perfection was lacking in the first, it is hard to imagine that a second world could have ‘patched up’ what was lacking in the first world in a significant way. Thus, a third world would need to be created to approach the perfection lacking in the set of worlds 1&2. However, this leads inevitably to an infinite regress, and perfection is never attained (unless infinite sets are possible and in this case actual). If it is gratuitous, it is hard to see why God would create two only if one did not suffice for God’s graciousness and purposes. Either this suggestion itself is incoherent, as a perfect God would need only one perfect world and thus not create another superfluously (as nothing he does is superfluous), or else it would need be again an infinite set of worlds that God created.
Either way, we are left with either some form of extreme modal realism as a theodicy, or else we are left with one single actual world and a traditional theodicy which denies the coherence of the best of all possible worlds.