Decision theory for agents with incomplete preferences

Orthodox decision theory gives no advice to agents who hold two goods to be incommensurate in value because such agents will have incomplete preferences. According to standard treatments, rationality requires complete preferences, so such agents are irrational. Experience shows, however, that incomplete preferences are ubiquitous in ordinary life. In this paper, we aim to do two things: () show that there is a good case for revising decision theory so as to allow it to apply non-vacuously to agents with incomplete preferences, and () to identify one substantive criterion that any such non-standard decision theory must obey. Our criterion, Competitiveness, is a weaker version of a dominance principle. Despite its modesty, Competitiveness is incompatible with prospectism, a recently developed decision theory for agents with incomplete preferences.We spend thefinal part of the paper showingwhyCompetitiveness should be retained, and prospectism rejected.