Orri Stefansson (Collège d’études mondiales, Paris): Desiring what one believes to be good

Abstract: The Desire-as-Belief thesis (DAB) states that a rational person desires a proposition exactly to the degree that she believes or expects the proposition to be good. Many people take David Lewis, the originator of the thesis, to have shown it to be inconsistent with Bayesian decision theory. However, as we show, Lewis’ argument was based on an Invariance assumption that itself is inconsistent with the Bayesian decision theory he assumed in his arguments against DAB. The aim of this paper is to explore whether arguments can be made against DAB without assuming Invariance. We first refute the standard version of DAB, which entails that there are only two levels of goodness. We next consider two theses according to which rational desires are intimately connected to expectations of multi-levelled goodness, and show that these are consistent with Bayesian decision theory as long as we assume that the contents of `value propositions’ are not fixed. We explain why this conclusion is independently plausible, and show how to construct such propositions.