Obj. 3:  Further, human nature is more self-sufficing than irrational creatures.  But irrational creatures have no Divine law besides the natural inclination impressed on them.  Much less, therefore, should the rational creature have a Divine law in addition to the natural law.

On the contrary, David prayed God to set His law before him, saying (Ps. 118:33):  “Set before me for a law the way of Thy justifications, O Lord.”

I answer that, Besides the natural and the human law it was necessary for the directing of human conduct to have a Divine law.  And this for four reasons.  First, because it is by law that man is directed how to perform his proper acts in view of his last end.  And indeed if man were ordained to no other end than that which is proportionate to his natural faculty, there would be no need for man to have any further direction of the part of his reason, besides the natural law and human law which is derived from it.  But since man is ordained to an end of eternal happiness which is improportionate to man’s natural faculty, as stated above (Q. 5, A. 5), therefore it was necessary that, besides the natural and the human law, man should be directed to his end by a law given by God.

Secondly, because, on account of the uncertainty of human judgment, especially on contingent and particular matters, different people form different judgments on human acts; whence also different and contrary laws result.  In order, therefore, that man may know without any doubt what he ought to do and what he ought to avoid, it was necessary for man to be directed in his proper acts by a law given by God, for it is certain that such a law cannot err.

Thirdly, because man can make laws in those matters of which he is competent to judge.  But man is not competent to judge of interior movements, that are hidden, but only of exterior acts which appear:  and yet for the perfection of virtue it is necessary for man to conduct himself aright in both kinds of acts.  Consequently human law could not sufficiently curb and direct interior acts; and it was necessary for this purpose that a Divine law should supervene.

Fourthly, because, as Augustine says (De Lib.  Arb. i, 5, 6), human law cannot punish or forbid all evil deeds:  since while aiming at doing away with all evils, it would do away with many good things, and would hinder the advance of the common good, which is necessary for human intercourse.  In order, therefore, that no evil might remain unforbidden and unpunished, it was necessary for the Divine law to supervene, whereby all sins are forbidden.

And these four causes are touched upon in Ps. 118:8, where it is said:  “The law of the Lord is unspotted,” i.e. allowing no foulness of sin; “converting souls,” because it directs not only exterior, but also interior acts; “the testimony of the Lord is faithful,” because of the certainty of what is true and right; “giving wisdom to little ones,” by directing man to an end supernatural and Divine.