Abstract: Sidgwick argued that utilitarianism was not rationally required because it could not be shown that a utilitarian theory of practical reason was better justified than a rival egoist theory of practical reason: there is a ‘dualism of practical reason’ between utilitarianism and egoism. In this paper, it is demonstrated that the dualism argument also applies to Kant’s moral theory, the moral law. A prudential theory that is parallel to the moral law is devised, and it is argued that the moral law is no better justified than this prudential theory. So the moral law is not rationally required. It is suggested that the dualism argument is a completely general argument that ethics cannot be rationally required.
Comment: This is a good and fairly accessible argument that casts doubt on the project of deriving morality from reason. It can be used alongside Kantian approaches to metaethics or reasons constituvism.