Part — Merkelbach, Summa Theologiae Moralis
Second Part: On Eternal Law Which Is the Foundation of Every Law
St. Thomas, I-II, q. 91, a. 1.
238. Thesis. Eternal law exists. - It is proved: law is an ordination of reason in the prince who governs a community. But, supposing that the world is governed by divine providence, the whole community of the universe is ruled by divine reason. Therefore the very ordering reason of the governance of things, which is in God as in the prince of the universe, has the notion of law. But divine reason conceives nothing in time but from eternity. Therefore such a law is eternal. Cf. Job XXXVIII-XXXIX; Prov. VIII-IX; Wis. VI-XII; Ps. CXVIII.
Nor should you say that it is not a law, since it has not been promulgated. For promulgation is not of the essence of law; moreover, although eternal law has not been passively promulgated on the part of the term or of the hearing creature (in second act), nevertheless it has been actively promulgated on the part of God promulgating (in first act), because it has everything which is required on God's part for promulgation, so that, once the creature exists, it is applied to this creature by that very fact. For it is not required for promulgation that the law become known, but that it be able to be known. Whence, just as human law obligates future persons because, by the very fact that they become members of the community, they fall under the law without a new promulgation, so eternal law, by the very fact that a creature exists, can be known by it and obligates it (ad 2). In other words: although promulgation has not been received, nevertheless it has been made.