Desire-as-belief revisited
On Humes account of motivation, beliefs and desires are very di¤erent kinds of propositional attitudes. Beliefs are cognitive attitudes, desires emotive ones. An agents belief in a proposition captures the weight he or she assigns to this proposition in his or her cognitive representation of the world. An agents desire for a proposition captures the degree to which he or she prefers its truth, motivating him or her to act accordingly. Although beliefs and desires are sometimes entangled, they play very di¤erent roles in rational agency.1 In two classic papers (Lewis 1988, 1996), David Lewis discusses several challenges to this Humean picture, but ultimately rejects them. We think that his discussion of a central anti-Humean alternative  the desire-as-belief thesis  is in need of renement. On this thesis, the desire for proposition p is given by the belief that p is desirable. Lewis claims that [e]xcept in trivial cases, [this thesis] collapses into contradiction(Lewis 1996, p. 308). The problem, he argues, is that the thesis is inconsistent with the purportedly plausible requirement that ones desire for a proposition should not change upon learning that the proposition is true; call this the invariance requirement. In this paper, we revisit Lewiss argument. We show that, if one carefully distinguishes between non-evaluative and evaluative propositions, the desire-asbelief thesis can be rendered consistent with the invariance requirement. Lewiss conclusion holds only under certain conditions: the desire-as-belief thesis conicts with the invariance requirement if and only if there are certain correlations between non-evaluative and evaluative propositions. But when there are such correlations, we suggest, the invariance requirement loses its plausibility. Thus Lewiss argument against the desire-as-belief thesis appears to be valid only in cases in which it is unsound.