G. E. Moore's ethical writings, especially Principia Ethica of 1903, have long been regarded as philosophically revolutionary. In fact, Moore shared his main ethical views—nonnaturalism in metaethics and ideal consequentialism in normative ethics—with such late-nineteenth-century writers as Henry Sidgwick and Hastings Rashdall. But Moore defended these views with unusual vigor and so had a disproportionate influence on later moral philosophy.

Moore's nonnaturalism comprised two main theses. One was the realist thesis that moral judgments are objectively true or false; the other was the autonomy-of-ethics thesis that moral judgments are sui generis, neither reducible to nor derivable from nonmoral judgments such as scientific or metaphysical ones. Our knowledge of them must therefore derive from intuitive judgments of self-evidence.

Moore did not argue extensively for realism. Like others of his era, he took it largely for granted. But his argument for the autonomy...