If, for instance, the reason why I ought not to lie were that God had forbidden it, because it pleased Him so to do, then it would follow that He might have refrained from forbidding it, had He so willed. Obligatio naturalis, on the other hand, is directa. I ought not to lie, not because God has forbidden it, but because lying is evil in itself. All morality rests on this, that we do what we do on account of the inner character of the act itself. Consequently what gives rise to morality is not the act, but the disposition from which its performance springs. If I do a thing because it is commanded or because it brings advantage, and if I avoid doing a thing because it is forbidden or because I would be a loser by it, there is no question of any moral disposition; but if I do a thing because it is absolutely good in itself, my disposition is a moral one. We ought, therefore, to do a thing not because God wills it, but because it is righteous and good in itself—and it is because it is good in itself that God wills it and demands it of us.
(Immanuel Kant, Lectures on Ethics, trans. Louis Infield, The Library of Religion and Culture, ed. Benjamin Nelson [New York: Harper & Row, 1963 (first published in 1930)], 22)