In modern ethical treatises we find hardly any mention of God; and the idea that if there really is a God, his commandments might be morally relevant is wont to be dismissed by a short and simple argument that is generally regarded as irrefutable. ‘If what God commands is not right, then the fact of his commanding it is no moral reason for obedience, though it may in that case be dangerous to disobey. And if what God commands is right, even so it is not God’s commanding it that makes it right; on the contrary, God as a moral being would command only what was right apart from his commanding it. So God has no essential place in the foundations of morals.’

The use of this argument is not confined to a recent or narrowly local school of philosophers; it was used by the British Idealists when they dominated British philosophy, and . . . it was used much earlier than that. Nor is its use confined to people who do not believe in God; on one occasion when I attacked the argument, my chief opponents were not atheists but professing Christians. This is not surprising; for the argument was used by Christians of an earlier generation, the Cambridge Platonists, as a stick to beat that dreadful man Hobbes with. . . . And they in turn got the argument from Plato’s Euthyphro.

(Peter Geach, “The Moral Law and the Law of God,” chap. 5 in Absolutism and Its Consequentialist Critics, ed. Joram Graf Haber [Lanham, MD: Rowman & Littlefield Publishers, 1994], 63-72, at 63 [italics in original; ellipses added] [essay first published in 1969])