Contemporary students of St. Thomas Aquinas are generally relieved to find him answering negatively a question of his time, whether the children of unbelievers should be baptized against their parents’ wish. The considerations in favor would have had some purchase on the medievals, if not on us: the children’s salvation is at stake, and whatever natural authority a natural parent has would seem to be superseded by the authority of God, our supernatural parent, from whom all legitimate authority derives.

Aquinas gives two arguments against the practice. The first argument simply contests the efficacy of the means. Children baptized against their parents’ wish would tend, upon reaching the age of reason, to apostatize under their parents’ influence, and that result would be worse than if they were never baptized in the first place. Fair enough, but the point, however well-taken, furnishes no argument against, say, abducting and baptizing the children of unbelievers.

The second argument is more encompassing and ambitious. Baptizing the children of unbelievers against their parents’ wish, Thomas says, “is against natural justice.”

For a child is by nature part of its father: thus, at first, it is not distinct from its parents as to its body, so long as it is enfolded within its mother’s womb; and later on after birth, and before it has the use of its free-will, it is enfolded in the care of its parents, which is like a spiritual womb, for so long as man has not the use of reason, he differs not from an irrational animal; so that even as an ox or a horse belongs to someone who, according to the civil law, can use them when he likes, as his own instrument, so, according to the natural law, a son, before coming to the use of reason, is under his father’s care. Hence it would be contrary to natural justice, if a child, before coming to the use of reason, were to be taken away from its parents’ custody, or anything done to it against its parents’ wish.

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Profound and suggestive as one might find the likening of parental care to a “spiritual womb,” I suspect many readers will find somewhat jarring the argument surrounding this intimate appellation.

For instance, what could justly be called the standard pro-life argument claims that a new human life begins at conception and that humans, even at the earliest stages of pregnancy, are distinguished from other animals by an innate capacity for rationality—their possessing a rational nature, whatever their current mental capacities happen to be. Aquinas seems here to deny both premises. The child in the womb, he says, “is not distinct from its parents as to its body.” And while it’s true that Aquinas’s premodern embryology was faulty, that won’t explain what he says here, for even after birth the child “differs not from an irrational animal” until the age of reason—perhaps as late as seven years. To top it off, he likens the human child to an ox. The salient point about the ox is that it is legally its owner’s instrument, to be disposed of according to his ends. The suggestion seems to be that parental rights are to be modeled on property rights.

On the surface, this passage is ugly and dehumanizing. Yet, in my view, Aquinas is here articulating—through a characteristically unhelpful analogy—a fascinating, plausible, and even familiar picture of the relationship between parent and child.

Let me explain.

Listening to Reason as to a Father

The age of reason plays an important role in the above argument, but what is it?

At the age of reason, the child comes into use of his distinctively human psychological powers. This may remind readers of the terminology of “personhood” and “moral status” in which ethics at the beginning and end of life are frequently discussed today. Does being a person depend on meeting some threshold of mental ability? Do human organisms become persons, and come to deserve our respect, only when they reach that level of mental ability?

No and no. Aquinas’s statement that the child before the age of reason “differs not from an irrational animal” is infelicitous, for it is true in one respect but not in others. As we shall see, one of Aquinas’s presuppositions is that even before the age of reason, the goods of the child are the rational goods specific to the human kind, of which the child is a member. The early child’s personhood is not in question, and there are more interesting reasons for Aquinas’s preoccupation with the age of reason.

He seems to me to have in mind a remark made by Aristotle early in his Nicomachean Ethics, which Aquinas knew well. In Chapter Thirteen of Book I, Aristotle is attempting to assimilate his threefold division of the parts of the human soul—into its vegetative, appetitive, and rational powers—to a related twofold division of the soul into its rational and irrational parts. The rational powers, clearly, belong to the rational part. And the vegetative powers, it may almost as easily be seen, do not, because nutrition and growth are involuntary.

The appetitive powers, though, manifest a kind of amphibiousness. Like the vegetative powers, they are simply distinct from the rational powers and therefore, in the most literal sense, irrational. Indeed, sometimes they “clash and struggle with reason,” as in the weak-willed person, who recognizes, by reason, what he should do while following his sense appetite in the opposite direction. Conflict exists also in the strong-willed man, in whom reason wins out over his desire for what he knows is wrong. And in the virtuous and the vicious, reason and sense appetite are in harmony, for the temperate enjoy acting virtuously, and the intemperate have no qualms about acting viciously.

Considerations such as these lead Aristotle to say that the part of the soul concerned with sense appetite, unlike that concerned with nutrition and growth, “participates in reason” because it can listen to reason “as to a father.” Aquinas, I think, had this remark in mind in writing the passage above.

The analogy invites us to compare the (dis)order in agents’ souls to the (dis)order in families (as Aristotle and Aquinas elsewhere compare the relations in families to polities). The sense appetites are likened to the children in a family. According to Aristotle’s account of habituation, children come to desire what is good by acting for the sake of what is good repeatedly. What leads them to do this, before they can see for themselves that those ends are good, is the direction of their parents. If all goes well, the children’s ends come to agree with their parents’.

But things can go wrong. The vicious person is like a family in which there is harmony only because the children have learned from their parents to be wicked. In the family analogous to the strong-willed person, the children obey their parents, but with complaint and pouting. The weak-willed person is like a family in which, to the parents’ shame, the children call the shots.

The Community of Parent and Child

So it is possible to model the harmony or disharmony in an individual agent on that of families. On my reading, Aquinas, in his argument about parental rights, is interested in flipping the analogy the other direction. He hopes to think about actual families in light of the relationship between reason and sense appetite in the individual agent.

Aquinas says that the child is “part of” his or her father. Made about the mother and the child in the womb, this claim would be comprehensible, but what could Aquinas mean by insisting that a four-year-old, say, is part of his or her father?

To understand Aquinas’s remark, we should think about the parts of humans generally. Consider a hungry human adult. Hunger is a sense appetite shared with animals. It is a kind of pain, which gives rise to a desire for food. The reason humans feel hungry is because they need to eat if they are to be healthy and continue in existence.

Whether one wants to eat and whether one should eat, though, are distinct questions. It is the role of reason to answer the latter. A hungry adult may decide not to act from hunger for any number of reasons: to diet, to develop virtue, or to save food for others. His hunger is merely a messenger; reason sits in judgment on the message. When the temperate person does eat, it is in accordance with reason’s command. This is the relationship, in a well-ordered individual, between reason and sense appetite.

That relationship parallels the relationship between a child before the age of reason and his parents. Human children have many needs but few capabilities. This is most obvious in pregnancy, when the child is literally enveloped in its mother’s body, from which it receives nourishment. After birth, children still cannot feed, move, clean, clothe, or change themselves. Their parents are, as it were, provident for their entire good.

When a child is uncomfortable, it cries: physiologists and parents can tell you that a child’s sobbing is designed to elicit a reaction from adults. It is literally painful to the ear, and no one wants to endure it long. Its purpose is to notify the child’s parents of his or her needs so that they may be satisfied, just as the pain of a damaged limb urges its possessor to address the wound.

The unity of parent and child, then, is functional and quasi-biological. Parents are naturally poised to take care of the needs of their children, just as individual agents are naturally poised to take care of their own parts. To make these observations is to make a point about metaphysical biology. One can imagine a child whose parents do not care for it, just as one could imagine a human who never learns a language. But a Martian anthropologist visiting earth would observe that parents are solicitous for their children’s needs in ways like those I have described, just as it would observe that human beings speak languages. These are facts about how humans live and flourish.

Borrowing Mom and Dad’s Reason

Children not only lack the power to satisfy many of their desires. They also lack the distinctively human and rational form of choice that Aristotle calls prohairesis. This is the sort of choice that the weak-willed person lacks. And this is true even though we would generally describe a weak-willed person’s execution of a vicious plan—say, buying a ladder and waiting for the opportune moment to seduce his neighbor’s wife—as involving deliberation and choice. Prohairesis is not merely choice but choice in accordance with one’s conception of what it is to live happily; the weak-willed person lacks such choice because his reason is not in command.

In lacking prohairesis, children are like irrational animals. They differ from irrational animals because their goods are the goods of humans, which include the goods of reason. Human children characteristically become adults who have some conception of living well. That conception can be right or wrong, and the possibility of weakness of will shows that getting it right is not sufficient for living well.

To act well, reliably, after the age of reason, the emotions need to be rightly habituated. Children who lack the use of reason plainly cannot guarantee that this habituation comes about. It is to be effected, normally, by parents. Sometimes hungry children need to be fed; other times, they are begging for sweets, and it is the parents’ reason that must judge whether it is wise, for their health and their growth in virtue, to indulge them. The child, as it were, borrows the reason of its parents. This renders habituation possible, for it helps children perform generous, patient, courageous, and temperate acts even when they lack the inclination to do so.

To sum up, then, Aquinas is not merely likening a child to an ox or comparing parental rights to property rights. He is formulating an analogy of proportionality. As an ox is the instrument of its owner by the civil law, so also a child is under the care of its parents by the natural law. The meaning of the terms shifts systematically across each dimension. Importantly, the measure for the use of an instrument is one’s own aims, whereas the measure for the care of a child is the child’s good—and the child’s good includes not just its immediate, animal needs but the goods of reason that it cannot access on its own.

Children are need-full and helpless. Thankfully nature has provided, by making children affectionate, credulous, and obedient, and by making parents responsive to their needs. In one sense, nothing is more mundane than these observations, on which the unity of parent and child is founded. But they are not, for that reason, unremarkable.