He is a leading defender of a non-naturalistic moral realism, holding that moral statements are not reducible to natural terms. For example the 'good' cannot be described in terms of pleasures and pains nor the conclusion of any of the natural sciences (physics, biology).
He is the author of three books, Whatever Happened To Good And Evil?, The Fundamentals of Ethics, and Moral Realism: A Defence. In addition, he is editor-in-chief of the Oxford Studies in Metaethics and has co-edited Reason and Responsibility: Readings in Some Basic Problems of Philosophy, an anthology covering many aspects of ethics with the late Joel Feinberg. He is also the editor of two Blackwell anthologies, Foundations of Ethics (with Terence Cuneo), and Ethical Theory.