It is no trick to review Thomas Aquinas’s famous definition of law from the Quaestio de Essentia Legis (ST I-II q.90). One can go through the various attributes of law before coming to Aquinas’s summation: “nihil est aliud quam quaedam rationis ordinatio ad bonum commune, ab eo qui curam communitatis habet, promulgata”—“it is nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated” (ST I-II q.90 a.4 co.). However, if one hastens toward that definition, one may well miss important aspects of Aquinas’s argument in support of it. In particular, one overlook what it means for a law to be an ordinance of reason.

Aquinas begins by saying that law is a rule and measure of human actions (ST I-II q.90 a.1 co.). Here he follows earlier writers like Isidore of Seville (cf. Etym. 5.10, 5.19–20). But the rule and measure of human actions is reason, the first principle of human actions (ST I-II q.90 a.1 co.). Indeed, one may say that actions are properly human only insofar as they are rational (ST I-II q.1 a.1). Aquinas tells us that “In unoquoque autem genere id quod est principium, est mensura et regula illius generis”—“Now that which is the principle in any genus, is the rule and measure of that genus” (ST I-II q.90 a.1 co.).

Here one must attend carefully to definitions (cf. In I Post. An. L.5). Henri Grenier, author of the influential manual, Thomistic Philosophy, tells us that “[a] principle is that from which a thing in any way proceeds” (Vol. 1, no. 217). Aquinas, commenting on Aristotle’s Physics, tells us that, when Aristotle talks about “principles”: “per principia videtur intelligere causas moventes et agentes, in quibus maxime attenditur ordo processus cuiusdam”—“by principle he seems to mean moving causes and agents in which, more than in others, there is found an order of some progression” (In I Phys. L.1). Elsewhere he says “Tria videntur de ratione principiorum esse: primum quod non sint ex aliis; secundum quod non sint ex alterutris; tertium quod omnia alia sint ex eis”—“Three things seem to belong to the very nature of principles. First, they are not from other things. Secondly, they are not from each other. Thirdly, all other things are from them” (In I Phys. L.10).

Aquinas goes on to tell us that “Sicut autem ratio est principium humanorum actuum, ita etiam in ipsa ratione est aliquid quod est principium respectu omnium aliorum”—“Now as reason is a principle of human acts, so in reason itself there is something which is the principle in respect of all the rest: wherefore to this principle chiefly and mainly law must needs be referred” (ST I-II q.90 a.2 co.). Aquinas shows that the first principle is the last end, which for human life is happiness (ibid.). Indeed, Aquinas, following Augustine, argues that happiness is the last end proper to man as a rational creature (ST I-II q.1 a.8 s.c. & co.; e.g., Augustine, De Trinitate lib. XIII, c.5). Aquinas demonstrates at length that perfect happiness cannot consist in wealth, honor, glory, power, or any other bodily good (ST I-II q.2 a.1–5). Neither can happiness consist of delight, even delight in the supreme good (ST I-II q.2 a.6). Happiness must be therefore a good of the soul (but not in the soul) and indeed the universal good, the object of all men’s desires—God (ST I-II q.2 a.7–8).

We understand better, therefore, Aristotle when he says that just laws are those that produce and preserve happiness for the political community (NE 5.1, 1129b19; In V Ethic. L.2). One can draw all manner of other conclusions from this. For example, “cum beatitudo consistat in consecutione ultimi finis, ea quae requiruntur ad beatitudinem sunt consideranda ex ipso ordine hominis ad finem”—“Since happiness consists in gaining the last end, those things that are required for happiness must be gathered from the way in which man is ordered to an end” (ST I-II q.4 a.3 co.). And we know that in this life only imperfect happiness, which requires all sorts of external goods, is possible (ST I-II q.4 a.7 co.).

But we do not need to get too far into those weeds. The important thing is to recognize the connections between happiness, reason, and law. More than this, as before, one must recognize that these connections are not merely accidental. Law is an ordinance of reason, which means that it is necessarily ordered to happiness. And happiness itself is not a meaningless concept, dissolved for the most part into relativism—each person defines it for him- or herself. We know what the most perfect happiness is (cf. ST I-II q.3 a.8 co.). We know, too, that “Quod autem dicitur maxime tale in aliquo genere, est causa omnium quae sunt illius generis”—“Now the maximum in any genus is the cause of all in that genus” (ST I q.2 a.3 co.). And so on and so forth.