Sidgwick's Dualism of Practical Reason, Evolutionary Debunking, and Moral Psychology

Sidgwick's seminal text The Methods of Ethics left off with an unresolved problem that Sidgwick referred to as the dualism of practical reason. The problem is that employing Sidgwick's methodology of rational intuitionism appears to show that there are reasons to favour both egoism and utilitarianism. Katarzyna de Lazari-Radek and Peter Singer offer a solution in the form of an evolutionary debunking argument: the appeal of egoism is explainable in terms of evolutionary theory. I argue that like rational prudence, rational benevolence is subject to debunking arguments and so problematic, but also - and more importantly - that debunking arguments are irrelevant in the debate over the dualism of practical reason on the view of reason and rational intuitionism that Lazari-Radek and Singer embrace. Either both egoism and utilitarianism are debunked, or neither are. If I am right, Sidgwick's dualism is left standing.