In a fine essay in Public Discourse last month, Kenneth Kemp responded to two recent articles by Dennis Prager in National Review Online in which Prager argued that, without a theistic foundation, there can be no objective morality. I agree entirely with Kemp on this: we can know that certain actions are objectively right and others objectively wrong, totally apart from the issue of whether God exists. Although I thus agree with Kemp, I write today for two reasons. First, I want to expand on Kemp’s explanation of how we attain moral knowledge. Second, I want to note some important qualifications to the assertion that our moral knowledge is logically independent of our belief in God.

Regarding the first point, Kemp rightly compares Prager’s view to Hume’s famous assertion that “Reason . . . is the slave of the passions.” Hume meant that, given an end, reason can determine which means will be effective in attaining that end, but that reason is powerless to determine which ends we should pursue and which we should avoid. People who, like Kemp and me, adhere to the Aristotelian-Thomistic tradition, and hold that human beings can rationally determine whether an action is right or wrong, must be able to explain exactly how reason determines which ends ought to be pursued and which ought to be avoided. Once we do this, the basic outline of morality is straightforward: morally right actions are the ones that reason determines are effective means to the proper end, and morally wrong actions are the ones that reason determines are in conflict with that end.

So how does reason identify the proper end or ends for human beings? The answer is that just as, given an end, reason can determine which means are effective means to attaining that end, so too, given a means, can reason determine the end or ends to which that means will be an effective means. Thus, confronted with an unfamiliar apparatus, an engineer can work out the purpose of the machine by examining its structure and seeing how it operates under different conditions. Similarly, a biologist who discovers a new protein in a living cell can, by examining its chemical structure and its interactions with other molecules in the cell, determine what its function is—that is, the end to which it is an effective means. In general, just as human reason is very good at means-ends reasoning (given the end, it can devise the means), so too is it very good at ends-means reasoning: given the means, it can work out what the end must be.

Eudaimonistic moral theory depends on the possibility of applying such ends-means reasoning to human nature. That is, given all we know about human beings, whether from biology, psychology, sociology, history, or otherwise, the eudaimonist asks to what end is human nature an effective means, or, equivalently, what is its function. Of course, this kind of inquiry does not proceed with deductive rigor, and there will be room for disagreement at the margins—which is just what we should expect, for, as Aristotle points out, morality is not mathematics, and it would be impossible to explain the high level of disagreement in morality if moral knowledge were like mathematical knowledge. That said, however, nearly all the philosophers who regard the end of human nature as the basis of morality agree that the end for human beings is something like rational activity in a community of persons committed to such activity on some joint basis. This end—usually called the final end in this context—is the good life or telos for human beings. Actions are morally good—or right—if they are ordered to achieving this end, morally bad—or wrong—if they are in conflict with it.

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Notice that God does not enter into this account. We can inquire into the end of human nature without opening the question of its origins. We simply take human nature as given and ask to what end it is an effective means. This inquiry brackets the question of whether human nature was created by God, is the product of blind evolution, or arose in some other way. We can proceed in this manner because whether something is an effective means to an end depends on the objective characteristics of the thing itself, not on whether some intelligent agent gave it those characteristics for some purpose or other. This is why an atheistic Neo-Darwinist can speak about biological structures having functions and serving ends without thereby conceding the existence of a creating God. The eudaimonist adopts a similar attitude when, without reference to its origins, he takes human nature as given and inquires to what end it is an effective means.

Some people recoil at considering human nature as a means to an end because they have ringing in their ears Kant’s famous injunction in his Categorical Imperative to treat human nature, whether in ourselves or others, always as an end and never as a means. This is nearly the exact opposite of the eudaimonistic approach, which is not surprising, since Kant self-consciously rejected eudaimonism in his Groundwork for the Metaphysics of Morals and set out to found morality on an entirely different basis. As an Aristotelian-Thomist, I think he was quite wrong about the foundations of morals, but I would note too that when eudaimonists say that human nature is a means rather than an end, they do not understand this statement the way Kant did. Eudaimonists mean that human beings are naturally ordered to a final end, the pursuit of which requires that we treat people in certain ways, not that we may use people for any purpose that strikes our fancy.

Now, on the basis of the idea that human nature implies a certain final end, which in turn determines which actions are morally right and which are morally wrong, we can get morality going. We can construct arguments about why murder, adultery, and theft are bad and why honesty, moderation, and courage are good. Indeed, we can do quite a bit of moral philosophy. But, in such a system, the nature of moral obligation—the meaning of should or ought when used in their moral sense—will be rather thin. When a man acts immorally, we can say that he does wrong, that he acts contrary to human nature, and that, to the extent of his wrongdoing, he makes himself a bad man and his life a bad human life. But that is about all we can say, and some people find this inadequate. They think that this does not quite amount to moral condemnation; it is more akin to saying that the man has made a kind of blunder. There is something to this, and it brings us to the question of how God figures in morality.

For, if we go beyond merely taking human nature as given and show that God exists and has created human nature, then we can reasonably conclude not only that human beings have a certain final end but that this end has been ordained by God and that he intends for us to attain it. This step leaves the content of morality unchanged but transforms the nature of moral obligation. Now when a man acts immorally, he not only acts contrary to his nature and makes himself a bad man; he also affronts God by his disobedience and sets himself in opposition to the divine will. With such assumptions in the background, it makes sense to say that the wrongdoer not only does wrong but sins, becomes not only a bad man but a wicked one. The assumption that God exists thus changes the meaning of should or ought, transforming it from a hypothetical imperative (“If you would reach your final end, do thus-and-so”) into something akin to a categorical imperative (“Thou shalt do thus-and-so”).

But God’s existence also affects the content of morality. For, as I said above, in determining the final end of human nature, we start by considering whatever we know about human beings. But human possibilities, and thus the human function, depend in part on the environment in which we live, on the furniture of the universe, as it were, and so our understanding of the final end will depend on what we think about the nature of the universe generally. Thus, if God exists, then human beings, and the universe in which we live, were intentionally created by God, and we must take account of these facts in our theory of the final end. For, if the end for human beings is, roughly speaking, knowing the truth together with other rational beings, then, if God exists, knowing him and being in the proper relationship with him become key aspects of the final end. Morally good actions then include worship, prayer, piety, and the pursuit of knowledge about God—none of which would be good actions, or even intelligible ones, if God did not exist. Hence, whether or not God exists greatly affects not only the nature of moral obligation but also the substantive content of the final end.

Finally, the existence of God affects morality in yet a third way if we make the even stronger assumption that God not only exists but also intervenes in human history. The exact consequences for morality will depend on what we think God has said and done, but in Catholic theology, for example, when God intervenes in human affairs, he elevates human nature above its natural level by grace and directs it to a supernatural final end—the beatific vision of God in heaven, which exceeds the capacity of human nature unaided by grace.

This supernatural final end does not conflict with the natural final end but includes and subsumes it. Hence, every action ordered to the natural final end is also ordered to the supernatural one, and every action contrary to the natural final end is also contrary to the supernatural one. But because the supernatural end is greater than the natural one, there are special actions that are not required by the natural end but are required by the supernatural one and that God, when he intervenes in human history, commands us to perform: these are actions falling under the supernatural or theological virtues of faith, hope, and charity. This supernatural morality thus requires much more of us than does natural morality.

For various reasons, our contemporary public discourse generally brackets the question of God’s existence and argues about morals on a methodically agnostic basis. Fortunately, this does not hamstring us in discussing most issues of public policy: questions such as whether targeted killings by drones are morally permissible, whether we should allow banks to become too big to fail, or whether we should have affirmative action in college admissions are logically independent of the existence of God and can be intelligently debated without reference to his existence.

Outside the public square, however, things are rather different. Anyone who thinks that God exists and gives the matter a moment’s thought will realize that his relationship with God is a very important part—indeed, the most important part—of his final end as a human being. Although there is thus a sense, very important to our public discourse, in which morality is independent of God, there is another sense, which is very important for human life, in which nothing is independent of God. As he says, I am the Lord God and there is no other.