Treatise on Law (Hackett Classics)

Summa Theologica, Questions 90-97: Aquinas' discussion of law in STI-IIqq90-97--a discussion actually extending thru the less studied qq98-105--has been justly admired by jurists & others not otherwise interested in his work. But it's shaped by his concern there (1) to present for beginner theologians an overview of the cosmos & of the vast sweep of creatures out from their divine creator & back to the same transcendent being as their ultimate destiny & (2) to synthesize the traditional vocabulary & classic theological sources on law. So prominence is there given the eternal law by which god governs even inanimate creatures (by the laws of physics etc.) & to the participation of natural moral law in that eternal law. But when he's free from these textbookish constraints he emphasizes that law's most essential feature is something which isn't true of the laws of nature (physics, biology etc.), namely that it's an appeal to the mind, choice, moral strength & love of those subject to the law: ScGIII cc.114-117--Cf. STI-II q.91 a.2 ad3.
Law is a plan for coordination thru free cooperation. The structure of things being as it is, the principles of practical reason & morality (natural moral law & right) can be understood, accepted & lived by, as fully directive in conscience, w/out needing to be regarded as (what they really are) an appeal from mind to mind, a plan–-freely made to be freely adopted--for integral human fulfillment. As the creator was in no way constrained to choose to create this universe as distinct from any other good possible universe, so human legislators have moral freedom to choose amongst alternative possible legal arrangements, making one set of provisions legally & (presumptively) morally obligatory by the sheer fact of adopting it–-that is, by what he calls their determinatio: I-II q.95 a.2; q.99 a.3 ad 2; q.104 a.1.