The below is excerpted from an article I am drafting called “The End of Marriage,” to be published in CUA’s Crosier student journal.
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The English political philosopher John Locke, in sharp contrast to Aristotle, does not rely on an account of natural genesis to explain the structure of the family or the marital bond. Rather, Locke’s “hypothesis” of the state of nature—that men are naturally in a state of perfect freedom and equality to act and control one’s person and property within the bounds of the natural moral law—leads Locke to define relationships or forms of society as various degrees of consensual limitation on personal freedom and equality. Just as the state of inertial motion (although never observed) allows natural science to explain actually observed motions in which bodies’ velocities are not constant and uniform, so also the state of nature (although never observed) allows Locke’s political science to explain actually observed social groups in which individuals are not perfectly free and equal in action and property rights.
Locke defines “conjugal society” in the following terms: “a voluntary compact between man and woman” consisting “chiefly in such a communion and right in one another’s bodies as is necessary to its chief end, procreation” that “draws with it mutual support and assistance, and a communion of interests too, as necessary not only to unite their care and affection, but also necessary to their common off-spring, who have a right to be nourished, and maintained by them, till they are able to provide for themselves.” (Second Treatise, §78) That is, the conjugal bond is tied by voluntary consent to the marital sexual right, upon which follows mutual support and assistance and shared interests. This bond is for the sake of procreation and raising of children, and, as Locke notes, unless the civil laws say otherwise, a marriage is not of its nature permanent. That is, once the children have reached their full maturity as individuals, the purpose of having the marital bond ceases because its end has been fulfilled.
Thus, while Locke would be able to explain why the marital union is exclusive (monogamous)—namely because the “right in one another’s bodies” cannot be effectually shared—he cannot explain the permanence of the union. This is because he errs when identifying the nature of marital consent. In a view that predates Locke’s, St. Thomas Aquinas points out that the marital consent is consent to the conjugal union itself, not to marital sexual rights (see Summa Theologiae, Suppl., q. 48, a. 1, also see q. 67, a. 1). This union, the conjugal union, exists for the sake of the family that is naturally its result, and hence consent to the marital right is implied in marital consent. However, once we say that the consent is consent to the union itself, it follows that this union must be permanent, because the family or blood relationship for the sake of which the marital union exists is permanent. Marriage is exclusive and permanent, then, because marriage is a comprehensive union of persons (in mind through friendship and virtue and in body through the sexual union), which is structured by nature and effected by consent, all of which is for the sake of a family. That it is comprehensive requires that it be exclusive, and it must be comprehensive for the sake of fully realizing a familial good. Finally, because it aims at realizing this familial good, it must also be permanent.
In other words, Locke cannot explain the permanence of marriage as a feature of its nature (and instead must attribute it to a conventional prescription by human laws) because he does not pay enough attention to the permanence of the family as a natural unit. Instead, the family is treated as a mere collection of individuals (some of them in full possession of their rationality and consequent rights, viz. adults, and some of them imprefectly such, viz. children) all of whom seek their individual freedom. Thus, man is defined not in terms of being a part of a whole community, but as an autonomous unit who, pressed merely by necessity and not a noble good, seeks community as a remedy, not as a fuller perfection. The family, then, becomes an ancillary mechanism in the production of individuals, which individuals are the immediate material condition for the State as citizens.