Thomas Aquinas describes law as "a certain rule and measure of acts whereby man is induced to act or is restrained from acting." [ Aquinas, Thomas. On Law, Morality, and Politics, 2nd Ed. Hackett Publishing Company, Inc. 2002. Question 90, Article 1.] Because the rule and measure of human actions is reason, law has an essential relation to reason; in the first place to divine reason; in the second place to human reason, when it acts suitably, it is in accordance with the purpose or final cause fixed in it by God.
Law is directed by its nature to the good, and especially to the universal or common good. [ Question 90 Article 3.] It is addressed not chiefly to private persons but to the whole people meeting in common or to persons who have command of the community as a whole. Promulgation is the application of the law to those whom it is applied and the communication of this law to them is essential to the nature of the law.
Aquinas recognizes four main kinds of law: the eternal, the natural, the human, and the divine. The last three all depend on the first, but in different ways. Were we to arrange them in a hierarchy, eternal would be at the top, then natural, then human. Divine law is not in conflict with natural law, but it reaches human beings by a different route, revelation.
Question 91 starts off in Article one with eternal law. It is not a human law but is created by God himself. Eternal law cannot be understood, it is everlasting, and unchangeable. Thomas says the whole world is ordered by the providence of God not with chaos. Humans do not have it in themselves to sustain this law without it passing away. Everything comes into passing and is created by a God and has a purpose.
Even though humans are not eternal, the forms of human beings are eternal in the mind of God. These forms find their location in divine reasoning. The whole world is a cosmos because it is divinely ordered. Man is not of measure, but of the measured. For...