Thomas Aquinas, answered from the texts and cited to the page.
The eternal law is nothing else than the type of Divine Wisdom, as directing all things to their due end.1 Consider what a governor is. In any who governs, there must pre-exist a plan of the order of those things that are to be done by his subjects, just as in any craftsman there pre-exists a type of the things to be made by his art.2 That plan, in the one who governs, bears the character of law.
Now God, by His wisdom, stands to all creatures as the artificer to the products of his art, and He governs all the acts and movements found in each single creature. Wherefore the type of Divine Wisdom, as moving all things to their due end, is law in the fullest and most proper sense — and since it is in God from eternity, it is the eternal law.1
Three objections press here and are worth meeting. The first is whether the eternal law is merely a type in the Divine mind, since Augustine speaks of God making each thing according to its type. The answer is that types, considered in relation to each particular thing, are many and distinct; but law directs acts toward the common good, and things that are in themselves different may be considered as one according as they are ordained to one common end.
The eternal law is therefore one, since it is the type of that order.1 The second concerns how we know it. No one can know the eternal law as it is in itself, except the blessed who see God in His Essence. But every rational creature knows it in its reflection, greater or less — for every knowledge of truth is a kind of reflection and participation of the eternal law, which is the unchangeable truth.
All men know it at least as to the common principles of the natural law; as to the rest, they partake more or less, and are accordingly more or less cognizant of it.3 We see the sun by its rays when we cannot see it in its substance; so too with the eternal law and its effects, according to what is written: the invisible things of God are clearly seen, being understood by the things that are made.3
The third is whether all law derives from it. Since the eternal law is the plan of government in the Chief Governor, all plans of inferior governors must be derived from it.4 The power of the second mover is derived from the first, for the second does not move except insofar as it is moved by the first — so too in governance. Hence Augustine's judgment holds: in temporal law there is nothing just and lawful but what man has drawn from the eternal law.
And where human law deviates from reason, it is not properly law at all but violence; it retains only the appearance of law.4 One further distinction is worth drawing, lest providence and the eternal law be confused. Providence is not the eternal law; it follows upon it. The eternal law stands in God as those indemonstrable principles of action stand in us upon which we base our deliberation and choice.
Providence is to the eternal law as a demonstration is to its first principles — it presupposes the law and proceeds from it, which is why one may attribute an act of providence to the eternal law without inconsistency, as every conclusion is attributed to the principles from which it flows.5
the type of Divine Wisdom, as moving all things to their due end, bears the character of law. Accordingly the eternal law is nothing else than the type of Divine Wisdom, as directing all actions and movements.Summa Theologica Part I-II, Prima Secundae
Just as in every artificer there pre-exists a type of the things that are made by his art, so too in every governor there must pre-exist the type of the order of those things that are to be done by those who are subject to his government.Summa Theologica Part I-II, Prima Secundae
every knowledge of truth is a kind of reflection and participation of the eternal law, which is the unchangeable truth, as Augustine says. Now all men know the truth to a certain extent, at least as to the common principles of the natural law.Summa Theologica Part I-II, Prima Secundae
Since then the eternal law is the plan of government in the Chief Governor, all the plans of government in the inferior governors must be derived from the eternal law... in temporal law there is nothing just and lawful, but what man has drawn from the eternal law.Summa Theologica Part I-II, Prima Secundae
the eternal law in God is not His providence, but, as it were, a principle of His providence; for this reason one can, without any inconsistency, attribute an act of providence to the eternal law in the same way that he attributes every conclusion of a demonstration to self-evident principles.On Truth, Quaestiones Disputatae de Veritate, pp. 270–271