Summa contra Gentiles3.114 Chapter CXIV. That laws are given by God to man

It is evident from this that it was necessary for man to receive laws from God. For, as we have shown, just as the acts of irrational creatures are directed by God, inasmuch as they belong to the species, so are man’s actions directed by God, inasmuch as they belong to the individual. Now, in so far as they are actions belonging to the species, actions of irrational creatures are directed by God by a certain natural inclination, which is consequent to the specific nature. Therefore in addition to this something must be given to man whereby he is directed in his personal actions. And this is what we call law.

Again. The rational creature, as stated above, is subject to divine providence, in such a way as to participate in a certain likeness of divine providence, inasmuch as it is able to govern itself in its own actions, and other things also. Now, that by which the actions of people are governed is a law. Therefore it was reasonable that a law should be given to man by God.

Besides. Since a law is nothing else than a reason and rule of action, it is reasonable that to those alone a law be given, who know the reason of their action. Now, this applies only to the rational creature. Therefore it was fitting that a law should be given to the rational creature alone.

Further. A law should be given to those in whom is the power to act or not to act. But this belongs to the rational creature alone. Therefore only the rational creature is adapted to receive a law.

Moreover. Since a law is nothing else than a reason of action: and the reason for everyone’s action is his end: everyone who is capable of receiving a law must receive the law from the one who guides him to his end: even as the inferior craftsman is guided by the master-craftsman, and the soldier by the commander-in-chief. Now, the rational creature obtains his last end in God and from God, as we have already shown. Therefore it was reasonable that a law should be given to men by God.

Hence it is said (Jerem. xxxi. 33): I will give My law in their hearts, and (Osee viii. 12): I shall write to him my manifold laws.