Starting your reading of the Summa Theologica with this question (II-Ie.90.1, if you’re keeping score) is not a terribly painful place to begin. His first question on the law is: “Whether law is something pertaining to reason?” In other words, is there something inherently rational and objective about the law, or is it arbitrary, or is it merely self-referential?
Remember, here, Plato’s insistence that justice (we’ll come back to how justice relates to law) can’t only be the limited, somewhat erratic, merely human kind of justice we strive for at our best – much less the “might makes right” kind of justice so often actually acted on. Remember also the status of “Law” in the Scriptures, as revealed by God for a certain end. This is the foundational heritage Aquinas receives, and he has no interest at all in changing it. What he’s trying to do is define it, using an Aristotelian language and framework.
First, the objections: “It seems that law is not something pertaining to reason.” (This is the wrong answer, but he tries to establish it first, as I explained about his method in class.)
Objection 1, from St. Paul: “I see another law in my members, etc.” He takes this quite literally: if law is “in” the members, it can’t be inherently rational, since reason is not anything “in” any bodily organ.
Objection 2, from the nature of reason: reason is composed of power, habit, and act (he’s taking this from Aristotle, of course). But law is not any of power, habit, or act, for the reasons he lists. So it’s not inherently related to reason.
Objection 3, from the nature of law: law moves its subjects to right action, that is to say, in their will. Therefore law pertains to will, not to reason.
None of these objections, probably, will strike us as particularly forceful or concrete. Frankly, I think he’s fishing for objections here, because, given what he’s already established all through the first part and most of the first-of-the-second part, it should be pretty obvious that law, taken as an ideal or type, is derived from the Logos, therefore it is indeed something pertaining to reason. Indeed, his “on the contrary” doesn’t even need to cite a more authoritative source than the Summa itself! But he’s trying to be completely consistent in his method, so he proceeds as usual.
In fact, Objection 2 is not that weak a counter-argument. If the nature of reason does not overlap with the nature of law at all, then how can they pertain to each other in any way? But of course, the rebuttal is also fairly self-evident: law acts on the subject through reason’s power (law limits it), habit (law forms it), and act (law regulates it).
His “I answer that” always introduces the meat of the argument. “Law is a rule and measure of acts, whereby man is induced to act or is restrained from acting…” with respect to some end (toward a good, away from an evil). But the nature of man is reason, and the end of man is toward perfect reason (he doesn’t say so here, but of course we know this is the Logos). So reason itself is the fundamental “rule and measure” of all man’s acts; therefore law, as part of that “rule and measure,” pertains to reason.
Then he replies to the objections. To objection 1, he distinguishes the literal “in” versus the metaphorical “in.” To objection 2, he rebuts as above, laying out with considerable precision exactly what parts of the rational faculties are involved, and how. This is pretty meaty, itself. To objection 3, he notes the rational nature of the will itself.
So we conclude, then, not only that the law pertains to reason, but also have an idea of how it pertains. It is referential to the Logos; that is, law is, in its own nature, divine, and we know it by means of “natural law” and “revealed law,” exactly parallel to “natural revelation” and “special revelation” of other aspects of the divine Will and Persons. Law is non-arbitrary, therefore, and has an objective content that can be known. (He’ll get into that much more in question 91.) Law acts on the will of the subject, both directing (“Hey, there, Will, you should eat vegetables before dessert.”) and limiting (“Will, if you grab that cookie, you’ll get 5 lashes!”).
Ultimately, all of this hangs on his definitions of what reason is (the Logos, and its reflections in the created world, both personally, in my reason, and impersonally, in the intelligibility of stuff), and what law is ("a rule and measure of acts"). His definition of reason is pretty secure. But do you agree with his definition of law? What other definitions are possible, and do they still lead to the same conclusion, or to a different one?