Many of you will know that I’m sympathetic to modal realism; I think it solves many more problems than it causes, especially for theists. I want to address here what I think is a mistaken objection to modal realism: the so-called problem of indifference. The objection is originally formulated in Robert Adams.
Indeed, if we ask, “What is wrong with actualizing evils, since they will occur in some other possible world anyway if they don’t occur in this one?”, I doubt that modal realism can provide an answer which will be completely satisfying ethically (Theories of Actuality, 1974).
If Lewisian modal realism is true, then, just as we exist and our worldmates exist, so does every other individual and person inhabiting every other world. They all exist even though they are not all actual; they all exist, though they are not all our worldmates; they all exist though they do not all exist in our world. And these existing, non-actual persons are all as concrete, sentient, caring, and rational as we are. What Adams is calling attention to is that, if all of these beings exist in the same sense that we do, then they all matter morally, just as much as you and I do. But if we consider (literally or unrestrictedly, now) every existing sentient being in deliberating about what we should do, Adams complains, we will always reach the conclusion that it just doesn’t matter what we do.
No matter what we do–whether we do good or evil–it does not matter to the overall value of the pluriverse (i.e. the overall value of all possible worlds). If we do good, some counterpart of ours in similar circumstances will opt to do evil. If we do evil, some counterpart of ours in similar circumstances will opt to do good. It will necessarily balance out; the pluriverse and all of it’s inhabitants will not be any better off, or any worse off, overall no matter what we do. The overall value of the pluriverse is unchangeable. So, what difference does it make what we choose to do? We are led to moral indifference.
Mark Heller thinks there’s more to this argument than an appeal to utilitarian intuitions (‘The Immorality of Modal Realism’, Philosophical Studies, 2003). The indifference argument is not merely suggesting that, if we are utility maximizers, then it does not matter what we do. Heller gives three progressively complicated examples to show this. I won’t go through them in detail. But the initial case involves Roz and Lisa in a situation where Roz can save Righty (who will otherwise drown) and Lisa can save Lefty (who will otherwise drown), but circumstances prevent them from saving both drowning children. They know this. Heller says, rightly I think, that if Lisa chooses to let Roz save righty, she has done nothing wrong. Similarly for Roz.
The Roz and Lisa case is designed to be analogous to an inter-world situation for modal realists. Suppose modal realism is true. If Roz is in world W and can save Righty, and her counterpart Lisa is in world W’ and can save Lefty, then it’s true, says Heller, that either Roz or Lisa fails to save her drowning child in her world. If Lisa saves Lefty in W’, then Roz will not save righty in W, and vice versa. For reasons given above, Heller urges that it seems permissible for Lisa to choose not to save Lefty in W’, so that Roz saves righty in W. Similarly for Roz in W. Heller thinks that’s a problem for modal realism: it makes it permissible for us, here in the actual world, not to save lives in such cases (and the problem generalizes to other immoral actions). After all, if we act immorally, no matter. Our counterparts will pull up the slack. Heller concludes that modal realism is false.
But I want to underscore where the proposed analogy fails. If Lewisian modal realism is true, and Roz saves righty in W, it is not true that any counterpart of Roz, in any possible world, cannot save her drowning child. Under modal realism, it is in fact true both that (i) each counterpart in each world can save her drowning child and (ii) some counterpart in some world will not save her drowning child. So, assume that Roz saves Righty in W. Go to any possible world you like that includes a counterpart of Roz in a similar situation: it is false that that counterpart cannot save her drowning child. She certainly can. Nothing Roz does prevents her counterpart from doing so. What Lewisian modal realism ensures is that some counterpart in some world will not (not that she cannot) save her drowning child in that world.
So the proper analogy is the following. Suppose Roz informs Lisa that she (Roz) will save her drowning child only if Lisa does not. And Lisa, equally perverse, informs Roz that she (Lisa) will save her drowning child only if Roz does not. Each could save her child no matter what the other does. It’s just a perverse fact that they won’t. So, each will save her child if and only if the other does not. Why would that affect what either one is morally permitted to do? It wouldn’t. So, there is no moral problem for modal realism. The problem of indifference is not a genuine problem at all.