The Catholic Church claims that its ethical teaching, especially on sex, is based upon natural law. I first show that natural law theories prior to the Middle Ages provide no authority for the Church's teaching on sex. I then examine Aquinas's teaching on natural functions and natural law in the two Summae. I suggest that he partly anticipates Enlightenment thinking about law and morals. I compare his theory of natural law with that of Germain Grisez and John Finnis. Finally, I examine the notion of a principle of practical reasoning and indicate how such principles could be formulated to correspond to elements in human nature.
