On one reading, Western civilization is best exemplified in the brief conversation between Cephalus and Socrates in Plato’s Republic. The venerable Cephalus, patron of the feast and authority of the home, offers hospitality to Socrates and they politely converse until Socrates asks for a definition of justice, at which point Cephalus conveniently remembers that he must leave to attend the sacrifices.

Allan Bloom declares that Cephalus dominates the home because of his status as father: “Age is his title to rule, as it is in almost all regimes governed by ancestral custom … [R]everence for age, and hence antiquity, is one of the strongest ties which can bind a civil society together.”

However, continues Bloom, if there is to be a “frank discussion about justice…reverence for custom must be overcome, and the philosopher must take the place of the father.” The authority of custom must be purged so that “reason, free of limiting prejudices, can begin the search for an understanding of justice which is not merely opinion.”

Western civilization so understood is defined by skeptical rationality—demanding to know the cause, to see some evidence, to hear the testimony, to know why something is the case—and Socratic skepticism dethrones the pseudo-authority of custom, tradition, or antiquity.

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Perhaps Judge Vaughn Walker, whose ruling in Perry v. Schwarzenegger overturned California’s Proposition 8, imagined himself a contemporary Socrates when he declared that “Tradition alone … cannot form a rational basis for a law,” and “the ‘ancient lineage’ of a classification does not make it rational.”

Tradition alone fails to provide a rational basis for law—isn’t Judge Walker Socratic in his impulse and spirit? One could even think him Thomistic given Aquinas’s conclusion that “law is a rule and measure of acts” insofar as law is reasonable. Law does not find its legitimacy in what we happen to do—tradition—but in reason, just like Judge Walker declares.

Not quite.

Aquinas maintains a dialectic between reason and tradition in the human law, writing that because human law is “a dictate of reason, whereby human acts are directed,” human laws can be changed either to strengthen reason and make institutions “less frequently deficient in respect to the common weal,” or because of the “changed condition” in which human acts are made. That is, laws can be changed to be made more reasonable or because of changing circumstances. Aquinas teaches that human law can be altered if it is irrational or unjust: “in human affairs a thing is said to be just, from being right, according to the rule of reason,” whereas at the point where a human law “deflects from the law of nature, it is no longer a law but a perversion of law,” and can be changed.

If we stopped there, it might seem that Thomas agrees that tradition fails to provide a rational basis, yet he also insists that “the mere change of law is of itself prejudicial to the common good… [H]uman law should never be changed, unless in some way or other, the common weal be compensated according to the extent of the harm done.” Such compensation may be some “very great and very evident benefit” or a remedy of the clear injustice of the existing law. So while believing the human law to be changeable, Aquinas also insists that revisions of law are always harmful to the common good in that “custom avails much for the observance of laws,” while a change of law diminishes “the binding power of the law…in so far as custom is abolished.”

When Aquinas says that “custom avails much for the observance of laws,” he recognizes that frequent and unpredictable revisions tend to render habitual obedience of the law less probable since citizens have reason to think that an edict might soon change. Changes of law create disincentives to obey the law. But his claim is more than this prudential realization, for Aquinas also says that action “done contrary to general custom, even in slight matters, is looked upon as grave.”

H. L. A. Hart, in The Concept of Law, distinguishes between a habit and a rule. Deviation from usual habitual behavior “need not be a matter for any form of criticism,” whereas “rule deviations are generally regarded as lapses or faults open to criticism.” Further, it is the deviation from a rule which is itself “generally accepted as a good reason” for making the criticism. Third, members of a social group need not have any interest in the habitual behavior of others in the group, “still less need they strive to teach or intend to maintain it.” A rule, on the other hand, is taught and maintained “in the criticism of others and demands for conformity made upon others when deviation is actual or threatened.”

When Aquinas says that deviation from custom “is looked upon as grave,” he indicates that custom is something more like a rule than a habit, and rules are thought to provide good reasons both to criticize rule-breakers and to take action to teach and maintain the rules over time. Given the rule-like nature of custom, we ought not be surprised when Aquinas claims that the “binding power of the law is diminished, in so far as custom is abolished,” for he has previously described law as binding because reasonable, and custom is thought to provide good reasons to bind the behavior of others in the social group. In linking custom with the binding force of rational law, which I’ve compared to a rule, Aquinas is forcing us to ask if custom, like reason and the law, can be binding. That is, can tradition alone provide a rational basis for law?

To be sure, Aquinas explicitly rejects the notion that custom can override the natural or divine laws. Custom and common sense can be nonsense and wicked—the antiquity of patriarchy, slavery, honor killings, and abortion in no way legitimizes those practices. But all law, he continues, “proceeds from the reason and will of the lawgiver” as manifested through speech or deeds. We commonly think that law is promulgated by speech, by some edict or code or regulation, but actions also promulgate the law, “especially if they be repeated, so as to make a custom,” for the repeated actions, just like speech, declare the reason and will of the lawgiver. Customary action—tradition—“proceeds from a deliberative judgment of reason” and has the force of law.

Further, in a regime where the people are free to make their own laws, as in a representative democracy, the customs of the people count “far more in favor of a particular observance, than does the authority of the sovereign,” for while an individual cannot make law, “yet the whole people can.” Custom is an expression of reason and will made by the whole people with their repeated actions over time, and since an expression of reason and will is a promulgation of law, custom is a promulgation of law, with the same binding force.

Of course, any human law, whether promulgated by the sovereign or by custom, can be irrational and in opposition to the natural law, in which case the unjust law is no law at all and has no binding force. This is true of any and all human laws, but the laws of custom are no more or less revisable than the law of a sovereign or legislature—or the ruling of a court. Custom can be irrational and reprehensible, as can a court decision; custom can be rational and praiseworthy, as can a court decision. But it is just simply not the case that custom or tradition alone per se fails to provide a rational basis for a law. Not only might custom provide a rational basis for a law, but a good custom is a rationally binding law.

And good custom, as a rationally binding law, cannot be changed without being prejudicial to the common good. A rational custom, like any human law, should, as Aquinas concluded, “never be changed, unless in some way or other, the common weal be compensated according to the extent of the harm done.”

And there is, to put it mildly, evidence that the harms of redefining marriage cannot be compensated.