No thinker in the Aristotelian-Thomistic tradition can dispute the relevance of human nature in sexual ethics—or in any aspect of ethics, for that matter. As Aristotle correctly asserted, ethics is about eudaimonia, or human flourishing. To flourish as a human being is to flourish with respect to the specific kind of nature we have. And yet, so-called “new” natural law theory is regularly criticized for denying human nature’s role in morality or even denying teleology.

Nothing of the sort is true.

The dispute between new natural law theorists and other natural law theorists within the Aristotelian-Thomistic tradition is not a dispute about the relevance of human nature to ethics or about teleology in nature more generally, but rather about the proper place of facts about human nature in practical reasoning. That such facts do have a place in our practical reasoning is undisputed. The question is what that place is.

Following Aquinas (Summa Theologiae, I-II, q.94, a.2), “new” natural law theorists such as Germain Grisez, John Finnis, Joseph Boyle, Robert George, Patrick Lee, and others, insist that practical reasoning always starts with one’s grasp of the intelligible point of some possible end or purpose as humanly fulfilling and, as such, choiceworthy. This apprehension of value is epistemologically (though not ontologically) prior to theoretical knowledge about the fact of human nature that grounds it. And this grasp of the point and value, and thus the choiceworthiness, of a particular activity or condition is not derived (epistemologically) from methodologically antecedent facts about nature. On the contrary, on this account, we only come to fully understand human nature through our prior grasp of the possible objects of choice that fulfill it—i.e., values. For instance, it is only through our grasp of the intelligible point of marriage—the value of marriage, understood as a uniquely comprehensive union of persons that is fulfilled by the having and raising of children together—that we fully understand the telos of our nature with regard to sexuality.

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By contrast, “old” natural law theorists such as Edward Feser think that ethical reasoning begins with factual premises about human nature—facts one identifies by “theoretical” reflection independently of one’s practical grasp of the intelligible point of acting for the sake of this or that end. On this view, we discover what is good for us by identifying the natural ends or functions of our faculties.

When it comes to sexual ethics, the major difference between these approaches is not in terms of conclusions, but rationales for those conclusions. For new natural law theorists, the moral evaluation of sex acts is always determined with reference to that basic form of human flourishing which is called marriage. For old natural law theorists, the morality of sex acts is determined with reference to the natural purpose of the sexual faculties.

In today’s essay I outline two new natural law approaches to sexual ethics, and explain the role of nature in those arguments. In tomorrow’s essay I will contrast the new natural law approach with the old natural law approach, exemplified in the work of Edward Feser.

Finnis on the Good of Marriage

The new natural law approach to sexual morality begins with an account of marriage as a basic human good, i.e., an activity or end that is choiceworthy precisely as a constitutive aspect of human well-being and fulfillment. As John Finnis writes in “Marriage: A Basic and Exigent Good,”

Marriage is a distinct fundamental human good because it enables the parties to it, the wife and husband, to flourish as individuals and as a couple, both by the most far-reaching form of togetherness possible for human beings and by the most radical and creative enabling of another person to flourish, namely, the bringing of that person into existence as conceptus, embryo, child, and eventually adult, fully able to participate in human flourishing on his or her own responsibility.

The two aspects of the good of marriage—interpersonal union and procreation—are inseparable from one another. What makes marriage “the most far-reaching form of togetherness” is that it involves a union not only of minds and hearts but also of bodies, and genuine bodily union is only possible due to the sexual-reproductive complementarity of a man and a woman. In other words, it is only in coitus that two human organisms can truly become bodily one, jointly coordinating for the single biological end of procreation (whether or not procreation is intended or actually results).

Since, then, practical reason grasps marriage, so described, as a good to-be-pursued (and what opposes it as to-be-avoided), sexual morality consists in determining which choices, acts and intentions are compatible with respect for the good of marriage. A sex act—any deed, word, or thought intended to cause sexual arousal and/or satisfaction in oneself or another—is wrong when and insofar as it is contrary to the good of marriage.

What makes a sex act contrary to the good of marriage? To actualize the good of marriage, a sex act must (1) fulfill the behavioral conditions for procreation (i.e. enable the couple to achieve a true bodily union through joint coordination toward the biological end of reproduction) and (2) be done with the right intention—namely, to actualize and express one’s marital commitment. Any sex act that does not fulfill these two conditions is contrary to the good of marriage.

It is worth pausing for a moment to explain further what these conditions mean and why they are necessary for achieving a marital act. Regarding the first condition, one might wonder why only coitus—the only sex act that fulfills the behavioral conditions for procreation—can actualize the good of marriage. The reason is that only through coitus can two human beings become biologically one—coordinating jointly toward a single biological end—because coitus is the only human act that is intrinsically oriented toward procreation. While other sex acts may produce pleasures or psychic satisfactions very similar to those produced in coitus, they do not actualize and express the good of marriage, because they do not enable the couple to achieve a real bodily union—become truly, and not merely metaphorically, “one flesh.”

The second condition is necessary because marriage is not only a union of bodies but a union of persons. Thus, bodily union has to be matched by a corresponding union at the level of mind and will. An act of sexual intercourse is a participation in the good of marriage only if done with the intention of actualizing and expressing one’s permanent and exclusive commitment to one’s spouse. Of course, other intentions and motivations (a desire to have children, a desire for pleasure, etc.) may or may not be present. What is necessary is that the intention to actualize and express one’s marital commitment be present as well.

For this intention to be present, all contrary intentions must be absent. This means, according to Finnis, that any willingness (actual or conditional) to engage in any non-marital sex act renders one’s own sex acts non-marital, even if engaged in with one’s spouse. For if one were at all open to the possibility of engaging in a sex act with someone other than one’s spouse, one’s intentions would lack the commitment to exclusivity and permanence that is required to make a sex act truly marital. Further, approval of someone else’s non-marital sex acts renders one incapable of participating in the good of marriage for the same reason, for it implies that there are some circumstances in which one thinks that non-marital sex acts are acceptable. In other words, engaging in, being open to engaging in, or approving of others’ engaging in any sex act that does not fulfill the two conditions set forth above sets one’s will contrary to the good of marriage, rendering one incapable of participating in that good until one repents, and is therefore morally wrong (because it is contrary to practical reason’s understanding that marriage is “to be pursued” and its opposite “to be avoided”).

Lee and George on Sex and Self-Integration

A complementary approach is offered by Patrick Lee and Robert George. They argue that to seek sexual pleasure apart from the genuine human good of marriage is wrong because (in addition to the points made by Finnis) it involves treating one’s bodily self (and that of one’s partner, if there is one) as a mere extrinsic instrument for the satisfaction of one’s conscious self. In a genuine marital act, the body is not used as an extrinsic instrument, because it is being used in pursuit of a genuine good in which the whole person (including the body) participates. In a non-marital sex act, by contrast, no genuine good of interpersonal communion is achieved, but only the illusion of such a good.

Lee and George articulate the wrongness of non-marital sexual acts not primarily in terms of their violation of the good of marriage (though the argument presupposes and makes reference to that good), but rather as failing to respect the integrity of the various aspects of the person (rational, emotional, and bodily). This line of argument is similar in many ways to the personalist account of sexual ethics set forth by Karol Wojtyla in Love and Responsibility and later in his works on the topic as Pope John Paul II. Along the same lines, Mary Catherine Geach, who is not a “new” natural law theorist, has argued that non-marital sexual acts are wrong because they involve “lying with the body,” “saying” one thing with one’s mind while contradicting that meaning with one’s bodily action.

The Role of Nature in New Natural Law Accounts of Sexual Morality

Note the ways in which facts about human nature are (and are not) employed in the above arguments. The arguments presuppose knowledge of the facts of human reproductive biology as well as a teleological understanding of coitus as an act that is ordered toward procreation, but it is not knowledge of these facts as such that grounds moral norms regarding sex. Rather, the arguments begin with a recognition of certain things as valuable in themselves, as constitutive aspects of human well-being.

We grasp the intelligible point of self-integration as something inherently choiceworthy. We recognize the intrinsic value of interpersonal communion and human life itself, including the transmission of human life through procreation. We grasp the value of marriage, both as a form of interpersonal communion unique in terms of both intensity and comprehensiveness and, inseparably, in terms of its relationship to procreation.

Theoretical knowledge of the natural biological ordering of our sexual faculties toward procreation does not help us to grasp this value or enable us to provide a stronger defense of it. A basic knowledge of biology, as well as decent exemplars of married life, can help us recognize the relevant possibility, but, as Aquinas makes clear (ST I-II, q.94, a.2), our grasp of the possibility as valuable is an original, self-evident (per se notum), practical insight. The insight that marriage as described above is an intrinsically valuable good to-be-pursued is a first (and, as such, immediate and underived) principle of practical reason, known to anyone who is aware of the relevant possibility and is not blinded by bias, partisanship, or some other sub-rational factor.

Having grasped this value at least in an inchoate way, we are then in a position to understand the fully human (biological, to be sure, but not just biological) purpose of our sexual faculties, and to understand marriage (and not simply biological reproduction) as the telos of the sexual aspect of our nature. This grasp of marriage as a basic human good is the principle that provides the rational foundation for the moral norms that govern our sexual conduct.

In claiming that we can only understand the various aspects of our nature by first understanding their objects, new natural law theorists are simply following the well-known Aristotelian axiom that a thing’s nature is revealed to us by its capacities, which are revealed by their actualizations, which in turn are revealed by their ends. And the ends of our nature in its various dimensions (biological, intellective, relational, moral, etc.) are precisely what new natural law theorists refer to as “basic human goods”: intelligible, intrinsic goods that, following Aquinas, practical reason self-evidently grasps as “to-be-pursued”—i.e., as basic reasons for action.

New natural law theory, therefore, in no way implies a denial of natural teleology on any level. What it does deny is that we can discover the natural ends of our human faculties through theoretical reasoning alone. Of course, we can know the biological ends of our biological capacities through theoretical reasoning alone, and knowledge of these ends provides important data for practical reason, because we are animal organisms. However, because we are not brute animals, but rational animals, the natural ends of our faculties (including our biological faculties) are not merely biological ends. Rather, they are intelligible goods that we discover through the insights of practical reason.

This essay is adapted from my article, “Sexual Ethics, Human Nature, and the ‘New’ and ‘Old’ Natural Law Theories,” in the Special Issue of the National Catholic Bioethics Quarterly, guest edited by Ryan T. Anderson, focused on new natural law theory.