Discourse about marriage is often short on reason and compassion. Arguments and objections can be mercilessly logical, forgetful of persons behind the positions; at other times reason seems to have taken a holiday, replaced with a kind of shrill indignation.

Perhaps failure is inevitable when marriage means too many things. Interlocutors cannot avoid speaking at cross-purposes when terms are in flux, and positions cannot but appear arbitrary if marriage is formless and mutable.

Given the confusion, it is understandable why the Harvard Journal of Law and Public Policy article “What Is Marriage?” by Sherif Girgis, Robert P. George, and Ryan T. Anderson drew so much attention, because they asked the marriage debate’s vital question. Definitions clear away weeds, clarity fosters productive disagreement, and subsequent rejoinders to the essay were numerous and pointed—a good sign that tackling the definition was needed.

Given the conversation stirred by the article, this winter’s release of What Is Marriage? Man and Woman: A Defense by Girgis, George, and Anderson is welcome (eBooks go on sale today; hard copies in early December). Augmenting and developing their earlier arguments, the book responds to objections, while tackling an impressive range of philosophical, legal, and social-scientific issues in a relatively slim volume.

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The authors explain why debates over gay marriage are only indirectly about homosexuality; the real issue is “not about whom to let marry, but about what marriage is.” Thus the need to address whether the conjugal or revisionist view of marriage is correct. Briefly, the conjugal view holds that marriage, by its nature, can only be the union of man and woman, because marriage is “distinguished by its comprehensiveness,” requiring “a bodily as well as an emotional and spiritual bond.” The revisionist view, on the other hand, treats marriage as a malleable union of any two partners; here marriage is simply “a loving emotional bond, one distinguished by its intensity.”

The book begins with a pointed challenge to revisionists: What does it mean to say that marriage is an intense emotional bond, and what does such a definition limit? The burden of proof is placed on the revisionists, and they do not fare well.

Take Oscar and Alfred as an example. They have lived together for some time, trust and love each other deeply, spend their time and affection especially with each other, and cannot imagine what life would be absent the other. But, are they married? Marriage may be an emotional bond of particular intensity, but of what sorts?

We tend to think that Oscar and Alfred are sexually involved, but need they be to meet the revisionist definition? What if they are bachelor brothers who’ve spent their lives together—are they married, and would it be unjust to deny them the status and benefits of married spouses? If not, why not? And what if Herman joins them? Whatever basis Oscar and Alfred had for their emotional bond, sexual or otherwise, is in principle open to the addition of Herman as well. Is this marriage? If, as revisionists claim, “marriage is distinguished simply by emotional union and the activities that foster it,” what rational basis limits marriage to exclusive monogamy while not including polyamory, close friends, or any sort of arrangement?

According to the conjugal view, on the other hand, marriage is more than a formless emotional bond; it is a basic human good, a way of being well with an objective and rational basis. Marriage cannot be redefined without deeply misconstruing (and thus missing out on) this good and the flourishing it partially constitutes.

The authors propose an analogy with friendship: “Suppose someone thought that friendship was mainly about people using each other, with occasional good will and cooperation just an optional spice.” Would this view offer a new form of friendship? Or would we conclude that someone with this opinion has not only failed to understand what friendship is, but is also not a friend? Concluding that most of us would take the latter stance, the authors suggest that this person “would not just be mistaken about the meaning of a word,” but would be mistaken about “a basic human good, one of the core ways of being well.

Marriage, like friendship, also has an objective core that cannot be altered without misunderstanding it. This core distinguishes marriage from other forms of relationship. As opposed to ordinary friendship, “marriage unites people in all their basic dimensions,” including minds, wills, and bodies. Since “your body is an essential part of you, not a vehicle driven by the ‘real’ you, your mind,” it follows that “any union of two people must include bodily union to be comprehensive.”

Thus a man and woman of deep intellectual and emotional commitment are not married without bodily union, and neither are Oscar and Alfred, the bachelor brothers. Likewise, same-sex partners or members of a polyamorous community cannot be married, because they cannot establish the bodily union that’s necessary for comprehensive union.

Critics often misunderstand this point: they confuse “bodily” union with united genitals or fused gametes. The book helpfully responds. Not really about genitals or their supposedly “natural function,” the argument instead concerns social action.

In marriage, as in a society, the members are united when they cooperate for a common purpose. Rocks in a pile lack this unity, but the organic systems of our body allow various organs to form one body. That illustration doesn’t quite count as social action, but it shows that unity obtains if two organs “coordinate toward one end that encompasses them both.”

In a similar way, for two people to be bodily united, “their bodies must coordinate toward a common biological end.” More than touching, however pleasant, and more than emotional bonding, however intense, “it is a remarkable fact that there is one respect in which this highest kind of bodily union is possible between two individuals, one function for which a mate really does complete us: sexual reproduction.” Our organic systems are perfectly sufficient in maintaining systemic union without the body of another except in sexual reproduction, and there, “and there alone,” the sexual difference of man and woman allows a coordinated activity toward a common end that neither can perform alone.

While not every marital act, or even every marriage, results in children, it is still true that the unity of mind and body that makes a marriage is “especially apt for, and enriched by, procreation and family life.” Marriages are the “main and most effective means” of raising flourishing children, and since the “health and order of society” depend on such children, the law has a compelling reason to support conjugal marriage.

While some on both the Right and the Left claim that marriage is malleable—something the state could remake to “fit our preferences”—marriage is not like other forms of friendship, which don’t need civil or legal structures. Only “male-female sexual relationships . . .  produce new human beings.” Raising such new human beings to maturity requires “a long and delicate process of ongoing care and supervision” if they are to attain maturity and “become healthy, upright, productive members of society.”

Still, while “civilization depends on strong marriages,” our affinities and inclinations may not always serve the genuine good, and a strong marriage culture is necessary to prompt and support this “costly and fragile” bond. Civil society bears a good deal of that weight, of course, but the state’s involvement “sends a strong public message about . . . what marriage is,” thus forming the expectations, beliefs, and behavior of actual marriages, especially in spousal commitment and support of children.

Marriage cannot be shuffled off as a private matter where individuals define their own commitments, for “at stake are rights, and costs and benefits . . . for all society.” Drawing on extensive research in the social sciences, the authors make a brief but impressive case for clear social benefits of marriage for children, including advantages in education, emotional health, teen pregnancy, sexual abuse, rates of delinquency, attention deficit disorder, and incarceration, among others.

Further, the benefits “exist primarily” for children of intact families with both biological parents, while children fare worse in every other alternative situation. To make marriage stable and supported “is to give more children the best chance,” and they should have “a right to their own two parents’ care.” Not children only, but spouses and families also benefit “financially, emotionally, physically and socially” from marriage, and as families go so too go economic and social stability, a strong but limited welfare state, and material well-being for the most vulnerable.

Still, we can expect a nonchalant shrug from some, even an admission that all this might be true and compelling, but why would legalizing same-sex marriage matter? Why not just let go?

The authors respond by explaining the pedagogical power of the law. Since law tends to shape beliefs, beliefs shape behavior, and beliefs and behavior affect human well-being, weakening the law makes marriage harder to realize. Even the revisionists show that they agree with this when they reject the adequacy of civil unions, knowing that what the law “calls a marriage” affects how citizens “come to think of marriage.” Consequently, redefining marriage would make a basic human good more difficult to achieve, for as marriage can be realized only by choosing it, a change of understanding would render choosing the objective, genuine good of marriage difficult, fragmentary, and confused: “people forming what the state called ‘marriage’ would increasingly be forming bonds that merely resembled the real thing.”

Further, as all spouses know, marriage is difficult, and our cultural norms that marriage should be monogamous, permanent, and exclusive lose their grounds if they are based merely on emotional bonds; this loss is championed by some leading (and consistent) revisionists. As norms wane, so too will the many benefits of marriage for children and spouses, and the already tenuous state of marriage would become “immeasurably harder to reverse.” In particular, men would lose the “social pressures and incentives . . . to remain with their wives and children,” or to marry prior to having children, even though the best evidence from the most well-documented studies shows the harms to children of lacking their fathers, including teenage pregnancy, sexual abuse, delinquency, and incarceration.

Freedom is also at risk. If the law withdrew its support of conjugal marriage, the state would “imply that the conjugal view makes arbitrary distinctions,” and supporters of conjugal marriage could not be viewed as anything other than unreasonable “champions of invidious discrimination,” with encroachment on conscience quite likely. Already defenders of the conjugal view are likened to racist supporters of antimiscegenation laws, and examples grow of their lost employment, eroding freedoms, and social stigmatization. Nor does the allegedly “conservative” claim that the revisionist view imposes marital norms on more relationships address that view’s arbitrariness or provide any reason why traditional marriage norms are binding once divorced from the conjugal view. The reality that they are not binding is supported with ample evidence of the high value placed on “openness” and “flexibility” among proponents of same-sex marriage.

None of the authors’ arguments suggests callous indifference to the well-being of those who cannot be married. As important as marriage is, human well-being entails many goods and many possible instantiations of those goods. The authors uphold the dignity of people in relationships that cannot truly be marriages, and argue that those persons deserve legal rights with respect to property, tax breaks, end-of-life issues, and so on, for “a policy that granted legal benefits to any two adults upon request,” whether they are sexually involved or not, “would offer no rival definition of marriage, or cause the harm of policies that do.”

In sections revealing their commitment to the “debate as . . . between people of sound mind and character,” the authors take care to explain how the conjugal view values non-marital friendship, encourages it, and even augments other relationships: “we all need community, and in this respect some people will know more hardship than others . . . People left dry by isolation of any kind we should graft onto other forms of community—as friends, fellow worshippers, neighbors, close partners in common causes . . . de facto members of our families and big siblings to our children, and natural and regular guests in our homes.”

There is no revulsion or bigotry evident here, but rather compassion for fellow human beings, and while reason reveals that marriage is conjugal and cannot be otherwise, reason—and compassion—demand caring for all, the married or the unmarried. But compassion does not, simply cannot, include redefining marriage, for marriage cannot be redefined. It is what it is.

What Is Marriage? Man and Woman: A Defense demonstrates a deep commitment to both reasons and persons in its substantial and calm argument. While there’s little cause to think that this fine text, perhaps the most thorough and accessible statement of the conjugal position, will end all debate, there’s good reason to think that it will occasion responses in kind, so that locked together in argument we may, all of us, pursue those common goods of truth and a just order.