Another World: An Argument From Natural Evil
Imagine a “twin Earth”, which is similar to our Earth, except that instead of being a planet with tectonic plates “floating” on a mantle, it’s solid rock with an iron core. This has a lot of biological, geographical, and meteorological implications, but imagine that the history of this twin Earth has played out roughly the same as our own. I don’t have space here to fully describe what such a planet’s history would look like (such a task would take several volumes!), but it is conceivable that there could be such a planet where evolution still happened, bipedal humanoids with roughly our characteristics still evolved, and there is still a global weather system that follows regular patterns.
The argument is as follows: a world which is similar to ours in all relevant moral characteristics is logically possible; and it is both logically and physically possible that this world still has weather systems which result in natural evils which are somewhat less in both frequency and severity. Perhaps the lack of tectonic plates results in no earthquakes or volcanoes, or perhaps tropical cyclogenesis results in smaller and weaker hurricanes.
If it is the case that such a world is possible, then it is reasonable to say that God, assuming he is in the business of actualizing worlds, would be morally obligated to create that world instead of this one. It might be possible to argue that some natural evil is necessary to achieve greater goods, and I won’t dispute that here. Even so, this argument asks the question, “Why is the amount of natural evil we actually have necessary for the correct amount of greater good?”
This twin Earth would have the same amount of good, but less natural evil. One might want to argue that less natural evil would mean less chances for higher goods, or less soul-making, but it remains to be seen why this is the case. People who are moved to do good by natural evil don’t seem to be, at least to a point, motivated by the amount of natural evil, but merely that it occurs. It seems at the very least unlikely that organizations such as Doctors Without Borders only exist because we have X amount of disease rather than X-1.
Of course, it is true that DWB probably wouldn’t exist if there was only one, or just a few, people in need of medical care. But we can still estimate a “fuzzy” lower limit on how much natural evil is needed before such things arise. Maybe it’s 30% of what we have now, or maybe 50%, or maybe 90% – but it seems quite unlikely that we’re currently at the bare minimum for these types of goods to exist.
A response to this argument might be something like, “well yes, these goods would still exist if there was less natural evil, but more evil means more good”. This can be responded to by flipping the argument around, and asking why there isn’t more natural evil in the world. And of course, it also seems highly unlikely that we’re currently at the upper bound of natural evil, after which any additional evil would not result in any more good.
 If you doubt this, simply consider a world in which we all have six fingers on each hand. Such a world would certainly be morally similar to ours.