Objection 1. It would seem that not every law is derived from the eternal law. For there is a law of the "fomes," as stated above (Question 91, Article 6), which is not derived from that Divine law which is the eternal law, since thereunto pertains the "prudence of the flesh," of which the Apostle says (Romans 8:7), that "it cannot be subject to the law of God." Therefore not every law is derived from the eternal law.
Objection 2. Further, nothing unjust can be derived from the eternal law, because, as stated above (2, Objection 2), "the eternal law is that, according to which it is right that all things should be most orderly." But some laws are unjust, according to Isaiah 10:1: "Woe to them that make wicked laws." Therefore not every law is derived from the eternal law.
Objection 3. Further, Augustine says (De Lib. Arb. i, 5) that "the law which is framed for ruling the people, rightly permits many things which are punished by Divine providence." But the type of Divine providence is the eternal law, as stated above (Article 1). Therefore not even every good law is derived from the eternal law.
On the contrary, Divine Wisdom says (Proverbs 8:15): "By Me kings reign, and lawgivers decree just things." But the type of Divine Wisdom is the eternal law, as stated above (Article 1). Therefore all laws proceed from the eternal law.
I answer that, As stated above (90, A1,2), the law denotes a kind of plan directing acts towards an end. Now wherever there are movers ordained to one another, the power of the second mover must needs be derived from the power of the first mover; since the second mover does not move except in so far as it is moved by the first. Wherefore we observe the same in all those who govern, so that the plan of government is derived by secondary governors from the governor in chief; thus the plan of what is to be done in a state flows from the king's command to his inferior administrators: and again in things of art the plan of whatever is to be done by art flows from the chief craftsman to the under-crafts-men, who work with their hands. 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. But these plans of inferior governors are all other laws besides the eternal law. Therefore all laws, in so far as they partake of right reason, are derived from the eternal law. Hence Augustine says (De Lib. Arb. i, 6) that "in temporal law there is nothing just and lawful, but what man has drawn from the eternal law."
Reply to Objection 1. The "fomes" has the nature of law in man, in so far as it is a punishment resulting from Divine justice; and in this respect it is evident that it is derived from the eternal law. But in so far as it denotes a proneness to sin, it is contrary to the Divine law, and has not the nature of law, as stated above (Question 91, Article 6).
Reply to Objection 2. Human law has the nature of law in so far as it partakes of right reason; and it is clear that, in this respect, it is derived from the eternal law. But in so far as it deviates from reason, it is called an unjust law, and has the nature, not of law but of violence. Nevertheless even an unjust law, in so far as it retains some appearance of law, though being framed by one who is in power, is derived from the eternal law; since all power is from the Lord God, according to Romans 13:1.
Reply to Objection 3. Human law is said to permit certain things, not as approving them, but as being unable to direct them. And many things are directed by the Divine law, which human law is unable to direct, because more things are subject to a higher than to a lower cause. Hence the very fact that human law does not meddle with matters it cannot direct, comes under the ordination of the eternal law. It would be different, were human law to sanction what the eternal law condemns. Consequently it does not follow that human law is not derived from the eternal law, but that it is not on a perfect equality with it.
Source: Summa Theologica (New Advent)