In his new book, One Body: An Essay in Christian Sexual Ethics, distinguished Baylor University philosopher Alexander Pruss offers novel defenses of traditional conclusions, combining insights from both new and old natural law theories of sexual ethics. He skillfully integrates scriptural revelation and contemporary sociological data in a philosophically imaginative and rigorous tour de force.

Pruss’s approach emphasizes that the fundamental law of life is love: love of God and love of neighbor. Therefore, it is essential that we properly understand the nature of love. Pruss challenges the quasi-canonical account of love—held by Anders Nygren among many others—that divides agape and eros as two distinct forms of love. Rather, he argues that agape is love itself, and all other divisions of love (e.g., brotherly love, erotic love, and parental love) are various forms of agape.

In all its forms, agape involves good will for the other, appreciation of the other, and seeking of unity with the other. Without appreciation, good will devolves into a condescending and cold bestowal of benefits. True appreciation of the good of the beloved leads naturally to a desire to be united in appropriate ways with the beloved. Indeed, all those who love enjoy at least a formal union if not a real union with those they love. The formal union (here Pruss draws on Aquinas) consists in understanding and also willing the good of the beloved as the beloved knows and understands his or her own good. A real union, which may or may not be achievable, includes various ways in which the lover and the beloved may be joined together. All love seeks real union, but all loves are not alike in the kinds of real union sought.

In Pruss’s view, the love of fiancés, the love of parent and child, the love of brother and sister, and the love of friends are not significantly different in terms of good will and appreciation. These distinct forms of love are differentiated mainly in terms of the kinds of union (consummation) that are appropriate to seek. Parents consummate their love for their infant children through baby-talk, swaddling, and bottle feeding or nursing. Scholarly colleagues may achieve a real union in their intellectual friendship through co-authoring an article. Spouses may achieve a real union through sexual intercourse. All love seeks real union, but we can differentiate various kinds of love (brotherly, spousal, erotic) by means of the diverse kinds of real union that they seek.

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Love becomes distorted when the reality of the lover, the beloved, and their relationship leads to forms of real union that are not fitting. As Pruss puts it:

If the nature of love calls for us to make the love take an appropriate form, then the form that the relationship should take is determined, at least in part, by facts outside the love itself. And this is how it must be since we need to appreciate the beloved as the beloved really is, to bestow things on the beloved that will really benefit him or her, and to unite with the beloved in reality. Thus, the characteristics of the beloved should have a role in determining the form of love.

A mother who loves her teenage son as if he were still in kindergarten has a disordered love, because her love does not sufficiently appreciate the reality of the teenager and seeks union in ways that are fitting only for a younger child. Since the law of love is the ultimate foundation of ethics, failures to love in ways that recognize the reality of the beloved, the reality of the lover, or the reality of the relationship are missing the mark morally. Thus, in Pruss’s view, human love is always dynamic because the realities of the beloved, the lover, and the relationship are always changing. What would benefit those we love (beneficence), the goodness of who they are (appreciation), and how we should seek union with them are not static—they are changing realities that call for our ongoing attention.

Erotic love or romantic love (here, I might quibble with Pruss in suggesting that these are not equivalent) is distinctive not in terms of good will or appreciation but in desiring sexual union with the other. And what exactly is sexual union? Sexual union as one flesh is a bodily union, and not just the union of any bodily parts. The union of a woman’s finger and a man’s ear is a bodily union of a kind, but it is not the bodily union which erotic love seeks. A real union as one body involves genuine coordination and a shared goal, rather than mere physical conjunction as could be achieved, for example, by gluing two cats together. In sexual intercourse, the body of one man and one woman are united. But Pruss asks: in what, exactly, does this bodily union consist?

There are three leading candidates for what constitutes the unity of two bodies: bodily pleasure, psychological unity, and reproductive striving. Pruss argues that bodily pleasure cannot be the goal of sexual union. While sexual union is widely recognized by almost everyone as tremendously significant, mere bodily pleasure does not seem a tremendously significant goal. Psychological union cannot fit the bill because other activities may bring about greater psychological union than some instances of sexual intercourse. Furthermore, sexual intercourse is about the body, not just the mind. Reproductive striving does bring about a real union because sexual intercourse is a shared bodily activity in which the bodies of both the man and the woman are striving toward the shared goal of reproduction, whether or not this goal is actually realized or desired.

On this basis, Pruss goes on to argue that all sexual activity that is not open to life is a violation of the ethics of love. For example, a woman using the contraceptive pill is intending to render sexual intercourse sterile, but at the same time her body and the body of her husband are seeking the goal of reproduction. So, in taking the pill for this reason, she undermines the real union that could have existed with her husband. In a similar way, the bodies of two men engaged in sexual activity are, on the biological level, seeking to emit reproductive seed by means of reproductive organs. But such a seeking is in vain, because the union sought in erotic love (a one-flesh union constituted by bodily striving for reproduction) simply cannot be achieved by two men physically conjoined in sexual activity.

One Body also takes up related questions such as the ethics of sex outside marriage, the use of reproductive technologies, deliberate sexual fantasizing, use of pornography, same-sex attraction, and celibacy. His conclusions on these matters provide a strong philosophical and theological defense of traditional teaching, often making use of empirical evidence joined by insights from faith.

In a future edition, I would like to see the book developed by making the section, “Is uncommitted sex morally acceptable?” into its own freestanding chapter. The organization could also be improved in other ways, perhaps following the example of Thomas Aquinas in the Summa contra Gentiles, by more clearly distinguishing the philosophical and theological aspects of the work. I worry that as it is presently written, some philosophers might not bother reading the book because they view it—wrongly, I think—as inextricably dependent upon an acceptance of revelation.

These quibbles do nothing to change the tremendous achievement of Pruss’s book. I can echo without reservation the words of Robert P. George, that One Body “is quite simply the best work on Christian sexual ethics that I have seen.”