A serious and thoughtful objection against metaphysical naturalism is that it cannot provide a basis for some of our deepest and most intuitive moral judgments. If so, a metaphysical naturalist could bite the bullet and say “so much for our deepest and most intuitive moral judgments!” Still, if this consequence could be avoided, it would remove a major stumbling-block for those who might otherwise view atheism as plausible.

The argument is clearly stated by Alvin Plantinga. He first notes that there seem to be instances of real and objectively horrifying evil in the world (Plantinga, The Shorter Routledge Encyclopedia of Philosophy, p. 326). The real and objectively horrifying acts that Plantinga means are those that are purposely and maliciously committed, like the hideous tortures and genocidal atrocities committed by Saddam Hussein, Stalin, the Nazis, or the Khmer Rouge under Pol Pot. We think that such acts are objectively horrifying, that is, that they would remain enormously evil even if all human beings could be somehow brainwashed into approving of them.

Plantinga comments:

Naturalism does not have the resources to explain or accommodate this fact about these states of affairs [that they are objectively evil and horrifying]. From a naturalistic point of view, about all we can say is that we do indeed hate them; but this is far short of seeing them as intrinsically horrifying. How can we understand this intrinsically horrifying character? After all, as much misery and suffering can occur in a death from cancer as in a death caused by someone else’s wickedness. What is the difference? The difference lies in the perpetrators and their intentions. Those who engage in this sort of thing are purposely and intentionally setting themselves to do these wicked things. But why is this objectively horrifying? A good answer (and one for which it is hard to think of an alternative) is that the evil consists in defying God, the source of all that is good and just, and the first being of the universe. What is horrifying here is not merely going against God’s will, but consciously choosing to invert the true scale of values…(Ibid., p. 326).

Plantinga says that it is not opposition to God’s will per se that makes an act intrinsically wicked and horrifying, but its intentional inversion of the true scale of values. But are theists the only ones allowed to posit a true scale of values? Why should this be so? Why cannot a metaphysical naturalist reasonably hold, e.g., that kindness is good and cruelty is bad, and would be so even if somehow all humans were perversely brainwashed into thinking that they weren’t? Plantinga seems to be assuming that a naturalist must be a subjectivist about values, that is, that for naturalists value is a function of how we feel about things. For the subjectivist, the only thing that can make cruelty bad is that we all feel a collective sense of horror or disgust when we contemplate cruel acts.

But naturalistic alternatives to subjectivism are well known, and have been for centuries. Aristotle articulated such an alternative 2400 years ago. In his Nicomachean Ethics Aristotle argued that the state of human well-being or flourishing, eudaimonia in Greek, is, objectively, the good for human beings. Eudaimonia is not made good by our feeling that it is. It would still be good if, perversely, all human beings were made to feel that it wasn’t. Neither is Eudaimonia made desirable by the fact that we do desire it; it would be desirable even if no human being desired it. Eudaimonia is best understood as a state of mental and moral excellence accompanied by a sufficiency of material and bodily goods. What makes such a state the good for man? Biology, says Aristotle—the biological nature of the human organism. Nature has designed the human organism to fulfill a characteristic function, just as other organisms are adapted to the performance of their roles in the economy of nature. Nature has therefore given humans the natural potentialities which, if actualized, will permit them to fulfill that function in an excellent manner. Humans are naturally adapted to live a life of intellectual and moral virtue in society with other human beings. This does not mean that most people actually live such a life; far from it. Most do not even perceive such a life as the natural end or goal of human striving, the state that is eminently and intrinsically worth achieving. Nevertheless, says Aristotle, it is in such a state, where human mental and moral capabilities are fully actualized, that humans will find the most fulfilling and rewarding life, for it is naturally pleasant to enjoy the use of our rational and moral faculties. Further, since most of our unhappiness in life springs from flaws in our own characters, particularly, as Aristotle notes, from our failure to form habits that avoid extremes of behavior (e.g., excessive anger or deficient generosity), the moderation entailed by virtue leads to a life of peace and equanimity even in the face of adversity.

Some have criticized Aristotle’s vision of the good life as elitist. Such critics say that it may spell out the best life for a well-off and well-educated Greek gentleman of the Fourth Century B.C.E., but its vision of the ideal life may be inappropriate to people of very different temperaments or cultural backgrounds. There may be something to such criticisms (not terribly much, in my opinion), but if naturalistic moralists can succeed in indicating, at least in broad outlines, the life that is intrinsically most rewarding and fulfilling for human organisms, then they will have identified something objectively valuable for human beings. That state will be objectively valuable in the sense Plantinga specifies, that is, it will be so even if everyone perversely identifies their well-being with something else, say, a life of hedonistic indulgence in sex, drugs, and rock-and-roll, or one of self-righteous indulgence in persecuting everyone who does not share one’s particular religious fixations. In other words, naturalists will have succeeded in identifying a set of objective values and can say, with as much justification as any theist, that some actions are laudable and others are horrifying because they tend either to promote or to thwart the realization of human well-being. If there is an objective state of human flourishing, then those who purposely and intentionally seek to impede the realization of that state in others, say by crushing the human body, mind, or spirit, are engaged in objectively horrifying and wicked acts. Those who do such things, to use Plantinga’s phrasing, are choosing to invert the true scale of values; they are preventing both their victims and themselves from attaining the natural end of human beings. So, metaphysical naturalism seems quite compatible with a moral view that recognizes certain actions as violently opposed to the true, objective scale of values, and hence as intrinsically horrifying.