Desire as Belief and Moral Newcomb Problems

Brian Weatherson (University of Michigan, Ann Arbor)

David Lewis's arguments against the Desire as Belief Thesis look like they should generalise to arguments against moral uncertaintism. By 'moral uncertaintism', I mean views that say we should treat moral uncertainty in roughly the same way that we treat factual uncertainty. I think moral uncertaintism is wrong, so it would be nice to have Lewis's arguments as extra ammunition. Unfortunately for me, Lewis's arguments against the desire as belief theory don't work. And thinking about how Lewis's arguments are incompatible with moral uncertaintism helps show why his arguments don't work. I think, though I won't really argue for the comparative claim in this paper, that this way of setting up the debate provides a much nicer response to Lewis's arguments than the vast majority of existing responses to him. But working through the desire as belief arguments from the perspective of moral uncertaintism reveals a distinctive kind of puzzle that arises within moral uncertaintism - what I'll call moral Newcomb problems. It turns out that just like orthodox decision theory splits into evidential and non-evidential versions, so does moral decision theory. And so it turns out there is an unexpected choice point within moral uncertaintism, and it isn't obvious which choice is the right one.