For such a certain knowledge of good and evil, we require moral principles that can be seen to be self-evident to us or natural moral laws of whose truths we can be certain. But since natural moral laws are only self-evident in themselves (assuming we know what that means) and since it is God’s reason and not man’s that is the source of the moral law, we poor mortals can have no rational certitude that the precepts claimed to be natural laws are really natural laws. [p. 201]
To discover what our natural inclinations are is simply to discover a fact about ourselves; to discover what purposes we have is simply to discover another fact about ourselves, but that we ought to have these inclinations or purposes or that it is desirable that we have them does not follow from statements asserting that people have such and such inclinations or purposes. These statements can very well be true but no moral or normative conclusions follow from them.
Natural law is often invoked in defense of Catholic doctrines, particularly when it comes to the Church’s positions on homosexuality and birth control. But what of the Protestant alternative? Unsurprisingly, Nielsen doesn’t think divine command theory – the view that good is what god commands, as god is himself the highest good – fares any better.
…a radically Reformationist ethic, divorcing itself from natural moral law conceptions, breaks down because something’s being commanded cannot eo ipso make something good. Jews and Christians think it can because they take God to be good and to be a being who always wills what is good. ‘God is good’ no doubt has the status of a tautology in Christian thought, but if so ‘God is good’ still is not a statement of identity and we must first understand what ‘good’ means (including what criteria it has) before we can properly use ‘God is good’ and ‘God is Perfectly Good.’
To treat the statement ‘god is good’ as an expression of identity would be to commit what G.E. Moore labeled the naturalistic fallacy. While this fallacy is often tossed about in criticisms of naturalistic ethics, there seems to be disappointingly little attention paid to the chapter on “Metaphysical Ethics” in the Principia Ethica, where Moore explains how it also applies to ethics founded on metaphysical truths, i.e. the existence of a god. Some theistic thinkers have taken this problem into account and argue that though good and god are not technically synonymous, there is nonetheless some relation between the two.
As Nielsen points out, however, this still leaves us without an understanding of what ‘good’ means. Even in tautological statements like ‘Wives are women’ and ‘Triangles are three-sided’, we know what women are and we know what it means to be three-sided. If ‘god is good’ is not an expression of identity, if it is not guilty of the naturalistic fallacy, then how are we to understand, much less believe, what is being asserted when we don’t understand what ‘good’ means? Nielsen puts it forcefully: “Morality does not presuppose religion; religion presupposes morality.”