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.