Listers, St. Thomas Aquinas asks the question Whether there was any need for a Divine Law? in his Summa Theologica I-II.91.4. The article is part of the Angelic Doctor’s treatment of law and more specifically the four laws that govern existence: Eternal, Divine, Natural and Human. In essence, the reason there was a need for the Divine Law - a law revealed by God - is because  Divine Law responds to a certain lack in and transcends the limits of man’s knowledge and naturally given end and capacity.

1. Eternal Happiness & Natural Capacity

“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 inproportionate to man’s natural faculty, as stated above (Question 5, Article 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.”1

2. Human Judgement Leads to Flawed Laws

“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.”

3. Man Cannot Judge the Heart

“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.”2

4. For the Perfection of Justice

“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.3 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.”4

  1. Grace: A common misunderstanding of grace is that it exist as an exemption from the law or allows us to bypass law - however, in actuality grace elevates the individual in order that he or she might live according to the law. ↩︎

  2. Interior Acts:  Divine law is capable of doing what natural/human law cannot do – it is capable of judging and guiding interior acts; “human law could not sufficiently curb and direct interior acts; and it was necessary for this purpose that a Divine law should supervene.” While man can judge man’s actions by natural and human law, divine law brings in an infallible judge – even in interior actions. ↩︎

  3. Imperfect Justice: ”Human law cannot punish or forbid all evil deeds,” thus that “no evil might remain unforbidden and unpunished, it was necessary for the Divine Law to supervene, whereby all sins are forbidden.” E.G. – look at the history of prohibition: an over zealous moral law led to the rise of the American Mafia. Divine Law – unlike Human Law – is capable of punishing evil without the exclusion of any goods. ↩︎

  4. Aquinas’ Summary of the Four Laws: “And these four causes are touched upon in Psalm 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.” ↩︎