• Melissa Moschella

Old Natural Law Theory, Marriage, and Sexual Ethics

“Old natural law theory” begins with the natural end of our sexual faculties and derives ethical principles from there. But this approach has to rely implicitly on prior value judgments in order to distinguish between biological facts that are axiologically or morally relevant and those that are not. The second in a two-part series.


In yesterday’s essay, I outlined the “new natural law” approach to sexual ethics, and explained the role that human nature and teleology play in that approach. Today, I contrast that approach with the “old natural law” account of sexual ethics, exemplified in the work of Edward Feser.

I argue that the old natural law approach has to rely implicitly on prior value judgments in order to distinguish between biological facts that are axiologically or morally relevant and those that are not. It has to appeal at least implicitly to values, not just facts, to ground the wrongness of immoral sex acts. Ultimately, it lends itself to a misunderstanding of the unitive aspect of marriage, since the good of marital union does not correspond directly to a biological end.

Feser’s Application of the Perverted Faculty Argument to Sexual Ethics What has come to be called the perverted faculty argument is as follows: Every act of preventing a human faculty or power from attaining its natural end is wrong. Contraception (or lying, or whatever) is an act of preventing a power from attaining its natural end. Therefore it is wrong. The basic idea is that human nature in this sense, including the various natural powers it includes, is the criterion for distinguishing right and wrong, therefore to act unnaturally is morally wrong. In “The Role of Nature in Sexual Ethics,” Feser argues that “the first step” in sexual ethics “is to identify the natural end or ends of our sexual faculties.” In order to explain what the ends of our sexual faculties are, Feser analyzes those faculties from both biological and psychological perspectives. He concludes that “a human sexual act is a seamless unity of the procreative and the unitive, directed at the same time toward both biological generation and emotional communion.” What follows is that “since the natural ends of our sexual capacities are simultaneously pro­creative and unitive, what is good for human beings vis-à-vis those capacities is to use them only in a way consistent with these ends”—i.e. to limit sexual activity to marriage, with openness to procreation. The key premise underlying these conclusions, according to Feser, is the perverted faculty argument, which forbids using one’s faculties contrary to their natural ends.

Feser is correct to say that our sexual faculties have both a unitive and procreative function. Yet the moral significance of this factual claim depends on our prior grasp of the values of human life and interpersonal—specifically marital—union. Feser has the order of derivation backwards.

It is not that a theoretical understanding of the telē (plural of telos) of our sexual faculties then leads us to conclude that those telē are intrinsically valuable. In fact, a complete theoretical understanding of the natural ends of our sexual capacities depends on our prior practical grasp of the human good that possession of those faculties enables us to achieve. Theoretical knowledge of biology can reveal the biological end of our sexual faculties, but it is only to the extent that we properly understand the good of marriage that we can come to a complete and accurate understanding of the rational human purpose of our sexual faculties.

Why Sexual Faculties Are Different It is only because we see the relationship between our sexual faculties and the good of marriage that we are in a position to recognize the moral restrictions on the use of those faculties. Indeed, with regard to the use of no other biological faculty are there such stringent norms.

I think Feser would agree that it is permissible to enjoy—even to the point of salivating at—the smell of fresh baked goods, even though one may not eat them because, say, they belong to another, and even when one is not trying to stimulate one’s appetite in preparation for a meal. That’s because, in doing so, one uses one’s nutritive faculty for something other than its natural purpose but not in a way that is contrary to that purpose. Yet if this is so, why is it not permissible (as Feser agrees it is not) to look lustfully at pornographic pictures, even if they are pictures of one’s spouse? These uses of the sexual faculties are no more or less contrary to the natural end of the sexual faculties than salivating at baked goods outside the context of a meal is contrary to the end of the nutritive faculties. Consider another example. Our sweat glands exist for a clear biological purpose of thermoregulation, and recent evidence indicates that the odor that results from our perspiration, particularly in the armpits, seems to have a biological purpose of sending pheromonal cues regarding the biological suitability of potential mates. Yet no one thinks that the use of antiperspirants and deodorants, which act contrary to these biological purposes, is morally wrong, even while one is intentionally engaging one’s sweat glands (by, for instance, sitting in a sauna). This is because acting contrary to these biological purposes does not entail a failure to respect an aspect of human well-being. There is no one-to-one correspondence between biological purposes and human well-being. Impairment of biological functions can be wrong if it is contrary to health, the preservation of life, or some other human good, but it is not wrong in and of itself.

The problem with Feser’s account is not with the basic principle that it is wrong to act contrary to one’s natural capacities. It is with his interpretation of the term “natural capacities” and his belief that purely theoretical knowledge can tell us what fulfills our natural capacities.

The principle is not true if the term “natural capacities” is understood as biological capacities and then applied to every system and subsystem within the body in isolation from the whole person, because the end of a particular system is not necessarily an end of the person as a whole. Of course, the end of each bodily system is related to integrated organic functioning, which is an end of the human person as a whole. Yet it is only the end of the person as a whole that is morally relevant in itself. Feser’s argument about sexual ethics makes the mistake of beginning with the ends of one bodily system (the reproductive system) and presuming them to be ends of the person as a whole. In this case, the conclusion happens to be correct, but that doesn’t make the theory or justification for the conclusion sound.

The Primacy of Practical Knowledge We do not discover what fulfills our natural capacities in the morally relevant sense by studying biology or metaphysics or anthropology. As Aquinas explained, just as all of our theoretical reasoning begins with first principles (such as the principle of non-contradiction), analogously all of our practical reasoning starts with its own first principles, principles that identify certain goods as to-be-pursued and their opposites as to-be-avoided. Such principles are per se nota and indemonstrabilia. They are not derived from methodologically antecedent knowledge of facts about human nature. They are not derived from anything. They are underived.

Consider the example of the intellect. How do we know that the end of the intellect is knowledge? Why not ignorance, or sophistry, or deception? None of these things would be possible, in the strict sense, if human beings lacked an intellect. Once we recognize knowledge and ignorance as possibilities, it is our practical insight into the (intrinsic) value of truth that enables us to then draw the theoretical conclusion that knowledge fulfills us qua intelligent beings and is therefore