 The most systematic presentation of Green’s idealist metaphysics and epistemology occurs in the first book of the Prolegomena. Green introduces his metaphysical and epistemological inquiries in response to a naturalistic approach to ethical theory that he thinks is prominent in British moral philosophy. He associates ethical naturalism with hedonism and focuses on the utilitarian tradition, especially John Stuart Mill and Henry Sidgwick (PE §2). Green associates naturalism with a kind of empiricism that attempts to construct knowledge on the basis of the senses, uncorrupted by the understanding. But Green thinks that there can be no empirical knowledge without operations of the understanding and concludes that a purely naturalistic or empirical science of morals is impossible (PE §8; cf. Works II 83–85).