All quotes from St. Thomas Aquinas, Treatise on Law, can be found here.
We have reflected before how law’s primary concern might be with the passions – that a law lasts depends on the sentiments people have toward it and what it represents; what a law does seems to involve the restraint of sentiment.
Now for Aquinas, “the extrinsic principle moving to good is God, Who both instructs us by means of His Law, and assists us by His Grace: wherefore in the first place we must speak of law; in the second place, of grace” (Summa, question 90, prior to article 1). Law is not based on a flighty or incomplete notion of the Good, but perhaps a complete one. We learn from Q 90, article 1 that
Law is a rule and measure of acts, whereby man is induced to act or is restrained from acting: for “lex” [law] is derived from “ligare” [to bind], because it binds one to act. Now the rule and measure of human acts is the reason, which is the first principle of human acts, as is evident from what has been stated above (Question 1, Article 1, ad 3); since it belongs to the reason to direct to the end, which is the first principle in all matters of action, according to the Philosopher (Phys. ii). Now that which is the principle in any genus, is the rule and measure of that genus: for instance, unity in the genus of numbers, and the first movement in the genus of movements. Consequently it follows that law is something pertaining to reason.
Do take note of where the idea of “it belongs to the reason to direct from the end” comes from – it comes from Aristotle, “the Philosopher,” and from his work the Physics. One thing I have encountered in studying for comprehensives is a thought by Harry Jaffa where he asserts that for Aristotle, each and every motion in the universe – not just for humans – can be discussed as action aiming towards a good. The end is indeed the “first principle in all matters of action” if that is the case.
One has to ask, though, why exactly one has to hold the universe as rational in order to truly understand the import of law. Our more modern perspective does not ultimately hold as Thomas does that “law is an ordinance of reason” which is “promulgated by the very fact that God instilled it into man’s mind so as to be known by him naturally” (Q 90, article 4), meaning that we do not hold that we can know the law by Reason, and we may not know by Reason a common good and what is essential to our happiness, either. Unity and a sense of what is primary may be intrinsic to numbers and movement, but not how and why we use law.
What is curious, though, is that the beginning of the modern understanding does not reject this sort of talk about reason. Locke in the Second Treatise says that the law of nature is reason, and that reason cannot be contradicted by the civil law. The trouble with the state of nature is that the law of nature is not respected there: the law is not “known,” as it is not “established” or “settled,” and passion clouds judgement, making application of the law quite problematic, and execution of the law is pretty uneven too (sections 124-126 of the Second Treatise). What is essential to the promulgation of law has less to do with force, and more to do with knowledge and judgment. So where does our more modern understanding, where we use law to impose order on a world which seems to be chaos, come from?
The answer is merely in the end of the law, for Locke, and how it differs from Aquinas: If Aquinas’ reasonable, natural law is ultimately subordinate to the grace of God, then Locke’s “reasonable, natural law” is tied to self-preservation and property rights (see section 123 of the Second Treatise). The same rhetoric is going two different directions, giving Aristotle, I think, the last say on this topic. For if even how we discuss law is subject to what we are passionate about, then the efficacy of law is indeed tied to passion more than reason.