Objection 1: It would seem that there is no eternal law. Because every law is imposed on someone. But there was not someone from eternity on whom a law could be imposed: since God alone was from eternity. Therefore no law is eternal.
Obj. 2: Further, promulgation is essential to law. But promulgation could not be from eternity: because there was no one to whom it could be promulgated from eternity. Therefore no law can be eternal.
Obj. 3: Further, a law implies order to an end. But nothing ordained to an end is eternal: for the last end alone is eternal. Therefore no law is eternal.
On the contrary, Augustine says (De Lib. Arb. i, 6): "That Law which is the Supreme Reason cannot be understood to be otherwise than unchangeable and eternal."
I answer that, As stated above (Q. 90, A. 1, ad 2; AA. 3, 4), a law is nothing else but a dictate of practical reason emanating from the ruler who governs a perfect community. Now it is evident, granted that the world is ruled by Divine Providence, as was stated in the First Part (Q. 22, AA. 1, 2), that the whole community of the universe is governed by Divine Reason. Wherefore the very Idea of the government of things in God the Ruler of the universe, has the nature of a law. And since the Divine Reason's conception of things is not subject to time but is eternal, according to Prov. 8:23, therefore it is that this kind of law must be called eternal.
Reply Obj. 1: Those things that are not in themselves, exist with God, inasmuch as they are foreknown and preordained by Him, according to Rom. 4:17: "Who calls those things that are not, as those that are." Accordingly the eternal concept of the Divine law bears the character of an eternal law, in so far as it is ordained by God to the government of things foreknown by Him.
Reply Obj. 2: Promulgation is made by word of mouth or in writing; and in both ways the eternal law is promulgated: because both the Divine Word and the writing of the Book of Life are eternal. But the promulgation cannot be from eternity on the part of the creature that hears or reads.
Reply Obj. 3: The law implies order to the end actively, in so far as it directs certain things to the end; but not passively--that is to say, the law itself is not ordained to the end--except accidentally, in a governor whose end is extrinsic to him, and to which end his law must needs be ordained. But the end of the Divine government is God Himself, and His law is not distinct from Himself. Wherefore the eternal law is not ordained to another end.