The Significance of the Dualism of Practical Reason

Sidgwick argued that utilitarianism and egoism were in conflict, that neither theory was better justified than the other, and concluded that there was a ‘dualism of practical reason’ and all that remained to him was ‘universal scepticism’. The dualism argument introduced by Sidgwick is an extremely powerful sceptical argument that no theory of ethics is rationally required: it cannot be shown that a moral sceptic or an egoist ought to accept the moral theory, otherwise she is unreasonable. I explain two ways in which the significance of the dualism argument has been underestimated. First, I suggest that a hybrid theory such as utilitarianism with an egoist bias is not (as is sometimes thought) a solution to the dualism. Second, I argue that the dualism argument is not restricted to a conflict between hedonic egoism and utilitarianism, but applies to any attempt to show that a theory of ethics is rationally required.