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There is a long, rich, varied, and subtle tradition of natural law theory, almost none of which I find especially convincing, but most of which I acknowledge to be—according to the presuppositions of the intellectual world in which it was gestated—perfectly coherent. My skepticism, moreover, has nothing to do with any metaphysical disagreement. I certainly believe in a harmony between cosmic and moral order, sustained by the divine goodness in which both participate. I simply do not believe that the terms of that harmony are as precisely discernible as natural law thinkers imagine.

That is an argument for another time, however. My chief topic here is the attempt in recent years by certain self-described Thomists, particularly in America, to import this tradition into public policy debates, but in a way amenable to modern political culture. What I have in mind is a style of thought whose proponents (names are not important) believe that compelling moral truths can be deduced from a scrupulous contemplation of the principles of cosmic and human nature, quite apart from special revelation, and within the context of the modern conceptual world. This, it seems to me, is a hopeless cause.

Classical natural law theory, after all, begins from the recognition that the movement of the human will is never purely spontaneous, and that all volition is evoked by and directed toward an object beyond itself. It presupposes, moreover, that beyond the immediate objects of desire lies the ultimate end of all willing, the Good as such, which in its absolute priority makes it possible for any finite object to appear to the will as desirable. It asserts that nature is governed by final causes. And, finally, it takes as given that the proper ends of the human will and the final causes of creation are inalienably analogous to one another, because at some ultimate level they coincide (for believers, because God is the one source in which both participate). Thus, in knowing the causal ends of nature, we should be able to know many of the proper moral ends of the will, and even their relative priority in regard to one another.

So far, so good. But insuperable problems arise when—in part out of a commendable desire to speak to secular society in ways it can understand, in part out of some tacit quasi-Kantian notion that moral philosophy must yield clear and universally binding imperatives—the natural law theorist insists that the moral meaning of nature should be perfectly evident to any properly reasoning mind, regardless of religious belief or cultural formation.

Thus, allegedly, the testimony of nature should inform any rightly attentive intellect that abortion is murder, that lying is wrong, that marriage should be monogamous, that we should value charity above personal profit, and that it is wicked (as well as extremely discourteous) to eat members of that tribe that lives over in the next valley. “Nature,” however, tells us nothing of the sort, at least not in the form of clear commands; neither does it supply us with hypotaxes of moral obligation. In neither an absolute nor a dependent sense—neither as categorical nor as hypothetical imperatives, to use the Kantian terms—can our common knowledge of our nature or of the nature of the universe at large instruct us clearly in the content of true morality.

For one thing, as far as any categorical morality is concerned, Hume’s bluntly stated assertion that one cannot logically derive an “ought” from an “is” happens to be formally correct. Even if one could exhaustively describe the elements of our nature, the additional claim that we are morally obliged to act in accord with them, or to prefer natural uses to unnatural, would still be adventitious to the whole ensemble of facts that this description would comprise.

The assumption that the natural and moral orders are connected to one another in any but a purely pragmatic way must be logically antecedent to our interpretation of the world; it is a belief about nature, but not a natural belief as such; it is a supernatural judgment that renders natural reality intelligible in a particular way. I know of many a stout defender of natural law who is quick to dismiss Hume’s argument, but who—when pressed to explain why—can do no better than to resort to a purely conditional argument: If one is (for instance) to live a fully human life, then one must . . . (etc.). But, in supplementing a dubious “is” with a negotiable “if,” one certainly cannot arrive at a categorical “ought.”

In abstraction from specific religious or metaphysical traditions, there really is very little that natural law theory can meaningfully say about the relative worthiness of the employments of the will. There are, of course, generally observable facts about the characteristics of our humanity (the desire for life and happiness, the capacity for allegiance and affinity, the spontaneity of affection for one’s family) and about the things that usually conduce to the fulfillment of innate human needs (health, a well-ordered family and polity, sufficient food, aesthetic bliss, a sense of spiritual mystery, leisure, and so forth); and if we all lived in a Platonic or Aristotelian or Christian intellectual world, in which everyone presumed some necessary moral analogy between the teleology of nature and the proper objects of the will, it would be fairly easy to connect these facts to moral prescriptions in ways that our society would find persuasive. We do not live in such a world, however.

It is, after all, simply a fact that many of what we take to be the plain and evident elements of universal morality are in reality artifacts of cultural traditions. Today we generally eschew cannibalism, slavery, polygamy, and wars of conquest because of a millennial process of social evolution, the gradual universalization of certain moral beliefs that entered human experience in the form not of natural intuitions but of historical events. We have come to find a great many practices abhorrent and a great many others commendable not because the former transparently offend against our nature while the latter clearly correspond to it, but because at various moments in human history we found ourselves addressed by uncanny voices that seemed to emanate from outside the totality of the perceptible natural order and its material economies.

One certainly may believe that those voices in fact awakened us to “natural” truths, but only because one’s prior supernatural convictions prompt one to do so. To try then to convince someone who rejects those convictions nevertheless to embrace those truths on purely “natural” grounds can never be much more than an exercise in suasive rhetoric (and perhaps something of a pia fraus).

The truth is that we cannot talk intelligibly about natural law if we have not all first agreed upon what nature is and accepted in advance that there really is a necessary bond between what is and what should be. Nor can that bond be understood in naturalistic terms. Even if it were clearly demonstrable that for the majority of persons the happiest life is also the most wholesome, and that most of us find spiritual and corporeal contentment by observing a certain “natural” ethical mean—still, the daringly disenchanted moralist might ask: “What do we owe to nature?”

To his mind, after all, the good may not be contentment or even justice, but the extension of the pathos of the will, as Nietzsche would put it: the poetic labor of the will to power, the overcoming of the limits of the merely human, the justification of the purely fortuitous phenomenon of the world through its transformation into a supreme aesthetic event. What if he should choose to believe (and are not all values elective values for the secular moralist?) that the most exalted object of the will is the Übermensch, that natural prodigy or fortunate accident that now must become the end to which human culture consciously aspires?

Denounce him, if you wish, for the perversity of his convictions. Still, after all hypothetical imperatives have been adduced, and all appeals to the general good have been made, nothing would logically oblige him to alter his ideas. Only the total spiritual conversion of his vision of reality could truly change his thinking.

To put the matter very simply, belief in natural law is inseparable from the idea of nature as a realm shaped by final causes, oriented in their totality toward a single transcendent moral Good: one whose dictates cannot simply be deduced from our experience of the natural order, but must be received as an apocalyptic interruption of our ordinary explanations that nevertheless, miraculously, makes the natural order intelligible to us as a reality that opens up to what is more than natural.

There is no logically coherent way to translate that form of cosmic moral vision into the language of modern “practical reason” or of public policy debate in a secular society. Our concept of nature, in any age, is entirely dependent upon supernatural (or at least metaphysical) convictions. And, in an age that has been shaped by a mechanistic understanding of the physical world, a neo-Darwinian view of life, and a voluntarist understanding of the self, nature’s “laws” must appear to be anything but moral.