As Augustine says, “There never seems to have been a law that was not just.” Thus it has the force of law insofar as it is just. In human affairs, something is said to be just insofar as it is right according to the rule of reason. The first rule of reason is, however, natural law. Thus every human law has the nature of a law insofar as it is derived from natural law. If it conflicts with natural law in any way, then it is not law but a corruption of law.
But it should be noted that something can be derived from the natural law in two ways: First, as conclusions from premises; second, as determinations of certain generalities. The first way is similar to that in which conclusions are demonstratively derived from premises in the sciences. The second is similar to the way general ideas are given special shape in the arts, as when a builder decides that he will actualize the general form of a house by constructing this or that particular model.
Thus some laws are drawn as conclusions from the general principles of natural law. For example, “you must not commit murder” can be derived as a conclusion from “you must do harm to no one.” Other laws are drawn from natural law by way of determination. For example, natural law requires that he who sins should be punished, but that he be punished by this or that punishment is a determination of natural law. Both are found in human law, but the first sort derive their strength, not only from the fact that they are legally enacted, but also from natural law itself. The second sort derive their strength only from human law.
Whatever is directed to an end should be proportioned to that end. The end of law is the common good, because, as Isidore says, “Law should be written, not for private gain, but for the general welfare of the citizens.” Thus laws should be proportioned to the common good.
This good is comprised of many things, and thus law should take many different persons, occupations and situations into account. A political community is composed of many people and its good is secured through many actions. Nor is it created to last a short time, but for a very long time and through generations of citizens, as Augustine says. — from “Aquinas on Law ” (Fordham University)