“We do not offend God unless we act contrary to our own nature.” This remark, which Thomas Aquinas makes in his book Summa Contra Gentiles, is a pithy summary of his view of morality. It encapsulates morality’s twofold source in human nature and God’s law. God commands us to act in accordance with the human nature that he created, so actions are specifically good or bad depending upon whether or not they perfect human nature, and therefore are reasonable for us to choose or avoid, respectively. Thus, in choosing well, we please God by our obedience, and in choosing badly, we offend him by our disobedience.

In our present intellectual climate, where rival atheist and theist camps disagree about whether God exists, why not circumscribe God’s role in this picture, bracket the question of his existence, and focus upon the ethical requirements of human nature alone? I want to consider a few reasons why this strategy is flawed, if it is adopted as a general method of ethics. It is, of course, possible to address many individual ethical problems in piecemeal fashion and on theologically neutral terms. There is no reason why vexed contemporary debates about abortion or gay marriage, for example, need to implicate theology. But the construction of an ethical theory, as a general matter, inevitably implicates what natural human reason can know about God.

This claim would appear to Plato, Aristotle, and Aquinas as obviously true. In the Nicomachean Ethics, Aristotle contends that the highest end of man is the contemplation of God. Even Plato’s Euthyphro, which is superficially interpreted by modern readers as critical of theological ethics, is in fact entirely consistent with a theological law conception of morality, as Elizabeth Anscombe argues in a posthumously published essay. For Aquinas, ethical theory necessarily implicates revealed theology in addition to natural theology, because he thinks that the final end of human beings is supernatural communion with God, which transcends the knowledge of God available through natural reason; the imperfect happiness afforded by merely natural goods is incapable of grounding an autonomous ethical theory. Even in the Summa Contra Gentiles, which does not presuppose the truth of biblical revelation, Aquinas suffuses moral discussions with natural theology. For example, Aquinas argues that the duty of human beings to act in accordance with their natures arises from the debt they owe God as their providential creator (see SCG II, 29, 17). In contemporary reconstructions of medieval natural law and classical virtue theory, however, God’s place in morality often becomes vacant or ambiguous.

Contemporary theorists hope that bracketing the question of God’s existence will clear the way for agreement on a conception of the natural world, which they assume will be less controversial than natural theology. But theists and atheists disagree about the existence of God because they already disagree about the nature of the world, including the nature of morality. To put it roughly, theists think that natural phenomena are not fully explicable apart from God, because they are finite and contingent. To explain why natural phenomena are what they are, and why they exist to begin with, it is necessary to appeal to an uncreated, necessary being—that is, to God. These natural phenomena include moral experience, such as the binding force of conscience, which is explicable only by inference to “the authority of divine precept,” as Aquinas argues. Atheists reject this inference, like the inference to a creator generally, because they assert that such ultimate explanation of nature is superfluous. There is no theologically neutral conception of the natural world that convinced theists and atheists share, therefore, because the disagreement about God’s existence, when it gets clear, is really a disagreement about how to characterize nature. For theists, nature’s finitude and contingency needs explanation, and for atheists, nature explains itself.

Let’s assume that the criterion of moral action is the final end, or ensemble of ends, that perfects human nature, and actions are good or bad insofar as they contribute to or frustrate the attainment of the final end. Actions will be intrinsically bad if they can never be ordered to the final end. At this point, the disagreement between theists and atheists about nature becomes morally acute. On atheistic assumptions, the final end must be some merely human good. Given this restriction, it is difficult to see how the final end could be specified determinately enough to demonstrate that certain kinds of injustice, such as intentionally killing the innocent, could never be chosen in order to advance the final end. Robert Miller, in his reply to my earlier essay on Public Discourse, says that this can be done, but he does not even hint at what the substance of the argument is supposed to be.

A lucid example that highlights this problem is John Rawls’s treatment of “supreme emergencies” in The Law of Peoples (1999). Rawls develops an account of just war that is similar to traditional theistic natural law theory, but different in a crucial respect: he allows for intentional killing of the innocent in supreme emergencies where the very possibility of civilized life hangs in the balance. The United States’ nuclear attacks against Japan did not meet this standard, Rawls argues, but the British attacks against Nazi Germany’s civilians earlier in the war did:

First, Nazism portended incalculable moral and political evil for civilized life everywhere. Second, the nature and history of constitutional democracy and its place in European history were at stake. Churchill really did not exaggerate when he said to the House of Commons on the day France capitulated that, “if we fail [to stand up to Hitler], the whole world including the United States … will sink into a new Dark Age.” This kind of threat, in sum, justifies invoking the supreme emergency exemption, on behalf not only of constitutional democracies, but of all well-ordered societies.

Whatever the merits of Rawls’s analysis of the historical examples, the principle of his argument is powerful—if his atheistic assumptions are granted. Rawls recognizes that intentional killing of the innocent is bad, perhaps intrinsically bad, and ordinarily should never be considered. The prudent statesman may nevertheless, in supreme emergencies, decide to kill innocent civilians, because Rawls’s political liberalism rules out trust in God’s promises or obedience to his commands as too sectarian for pluralistic politics. (Never mind Rawls’s sophistical appeal to “public reason,” elsewhere in the book, that allows him to exclude philosophical theology.) I simply want to point out that the non-theological arguments for moral absolutes do not hold up against a position such as Rawls’s. Early in the war, Churchill may have reasonably thought that he faced a tragic dilemma: kill some innocent Germans in the course of defeating the Nazis or let the Nazis kill countless innocent people everywhere. If the traditional prohibition against killing the innocent is to be compelling in such a situation, then the blinkers of methodological atheism must be removed.

Consider Peter Geach’s argument in his essay “Moral Law and the Law of God,” which I alluded to in my previous essay. Geach points out that non-theological arguments for exceptionless moral prohibitions, such as Aristotle’s, fail to show that it is unreasonable for someone to sacrifice his own moral integrity by doing something he takes to be bad if the action will seriously benefit others; Churchill could have plausibly thought that he was in this position. But suppose that natural reason can establish that God exists and, like human beings, is a voluntary agent, as many philosophers have thought. From this premise, Geach argues that it is reasonable to conclude “that God will not only direct men to his own ends willy-nilly like the irrational creatures, but will govern them by command and counsel.” Now given that we can know that practices such as intentional killing of the innocent are generally bad, we should infer that this knowledge “is itself a promulgation of the Divine law absolutely forbidding such practices.” This inference must be valid, because otherwise God would have created most people without promulgation of his commands about acts that they must never choose, which would contradict the initial conclusion that God is a voluntary agent who commands and counsels his creatures.

Geach’s argument needs to be developed at much greater length, of course, but it strikes me as promising; in any case, I have already pointed out flaws in Christopher Tollefsen’s attempt to justify exceptionless moral norms by appeal to the logic of “basic goods.” Tollefsen’s reply to my critique leaves the substance of my argument untouched. First, he repeats the common mistake that Anscombe dispatched in her famous essay “Modern Moral Philosophy”: divine law cannot generate a moral obligation unless there is already a moral obligation to obey divine law. This is like saying that a doctor can’t give you a prescription unless there is already a prescription for you to go to the doctor. On the contrary, for traditional theists such as Aquinas, a divine command gives you a new reason for doing what the command requires, because God has legitimate authority. His commands make perspicuous to you those acts and omissions without which you cannot reach your final end.

Second, Tollefsen misunderstands what tragic dilemmas are. They need not be cases where two moral absolutes conflict. They can be cases where a defeasible moral duty conflicts with an absolute prohibition. Rawls’s analysis of Churchill’s position early in the war provides just such an example: Rawls doesn’t claim that Churchill’s duty to protect the United Kingdom and her democratic allies was absolute; presumably this duty could be overridden in different circumstances. Rather, his argument is that this duty, although defeasible, justifies the intentional killing of the innocent in the circumstances of World War II, given what was at stake. The route to refuting Rawls is blazed by Aquinas, and it requires establishing that the universe, and man’s place within it, is providentially ordered, which would imply that “tragic” dilemmas are avoidable because they are either merely apparent or the consequence of prior wrongdoing.

What I have been arguing for here is an approach to ethical theory that is broadly in the tradition of Plato, Aristotle, Aquinas, Geach, Anscombe, and others who recognize that God’s existence is directly relevant to both the force and content of the claims that morality has upon us. Furthermore, this tradition also recognizes that the criterion for moral action is human nature and the goods that perfect it. The crucial question is whether or not human nature is created, for if it is, then the reasonable norms of nature are really obliging laws in a providential universe governed by the commands and counsels of the Creator. As John Finnis rightly points out in his book Aquinas, conceiving of human nature as created by God “enhance[s] both the content and normativity” of morality. His mistake, which Tollefsen echoes, is to think that the force of exceptionless moral norms can be demonstrated before broaching philosophical theology. The debate over moral absolutes is only one among a number of problems that illustrate the implications of how the question of God’s existence gets answered.

Matthew B. O’Brien is a postdoctoral fellow at Villanova University.

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