Bob Fischer

If David Lewis's modal realism is true, then there are many, many people. According to Mark Heller, this is bad news. He takes modal realism (MR) to imply that "there are at least some cases in which it is permissible to let drowning children drown when it would be easy to save them." But since he holds the reasonable view that this is never permissible, he thinks that MR is false. Here, I argue that Lewis needn't be troubled by this objection, and that it provides no reason to reject MR for those who share Lewis's moral outlook. Moreover, I argue that disagreement with common sense needn't be severe if we can show both (a) that there's a sense in which common sense is correct and (b) we have little reason to care about the sense in which common sense is mistaken.

Some philosophers think that a person can justifi ably believe that p is possible even though she has no theory according to which p is possible. They think, for example, that she can justifiably believe that there could be naturally purple elephants even though she lacks (inter alia) a theory about the factors germane to elephant pigmentation. There is a certain optimism about this view: it seems to assume that people are fairly good at ferreting out problems with proposed modal claims; so, if a person doesn’t detect one, then she’s within her rights to assume that there is no problem to be found. I am suspicious of this optimism, but I do not have space to challenge it here. Instead, I want to outline the more pessimistic alternative. Suppose that a person is not justified in believing that p is possible unless she has a theory according to which p is possible. How might such an account be developed? My aim is to map out this neglected option.