This means that the Divine law is in some instances promulgated to all men of sound understanding. No man can sincerely plead ignorance that lying, for example, is generally objectionable. I am not saying that a sane and honest man must see that lying is absolutely excluded; but he must have some knowledge of the general objectionableness of lying, and this is in fact a promulgation to him of the Divine law against lying. And he can advance from this knowledge to recognition of the Divine law as such, by a purely rational process.
To make this point clearer, let us consider a modern ethical philosopher who says ‘I do on the whole object to lying, but this is just a practical attitude I take up—it is quite wrong to call it “knowledge”’. I do not say of him what I should of a man who professed to have no special objection to lying: that he is just a vicious fellow, or a fool talking at random, who deserves no answer. What I do say is that his very protest shows that he does possess that sort of knowledge which is in fact God’s promulgation of a law to him. His erroneous philosophy will not allow him to call it knowledge; but that does not prevent it from being knowledge—philosophers in fact know many things that their own theories would preclude them from knowing. And since he has this knowledge, he has had God’s law against lying promulgated to him, even if he does not believe in God.
Thus, whatever a man may think, his rational knowledge that it is a bad way of life for a man to be a liar or an adulterer is in fact a promulgation to him of the Divine law; and he is able to infer that it is such a promulgation if he rightly considers the matter.
(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 69 [italics in original] [essay first published in 1969])
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