Reply to Objection 3: Even irrational animals partake in their own way of the Eternal Reason, just as the rational creature does. But because the rational creature partakes thereof in an intellectual and rational manner, therefore the participation of the eternal law in the rational creature is properly called a law, since a law is something pertaining to reason, as stated above (Q[90], A[1]). Irrational creatures, however, do not partake thereof in a rational manner, wherefore there is no participation of the eternal law in them, except by way of similitude.
Whether there is a human law?
Objection 1: It would seem that there is not a human law. For the natural law is a participation of the eternal law, as stated above (A[2]). Now through the eternal law "all things are most orderly," as Augustine states (De Lib. Arb. i, 6). Therefore the natural law suffices for the ordering of all human affairs. Consequently there is no need for a human law.
Objection 2: Further, a law bears the character of a measure, as stated above (Q[90], A[1]). But human reason is not a measure of things, but vice versa, as stated in Metaph. x, text. 5. Therefore no law can emanate from human reason.
Objection 3: Further, a measure should be most certain, as stated in Metaph. x, text. 3. But the dictates of human reason in matters of conduct are uncertain, according to Wis. 9:14 : "The thoughts of mortal men are fearful, and our counsels uncertain." Therefore no law can emanate from human reason.
On the contrary, Augustine (De Lib. Arb. i, 6) distinguishes two kinds of law, the one eternal, the other temporal, which he calls human.
I answer that, As stated above (Q[90], A[1], ad 2), a law is a dictate of the practical reason. Now it is to be observed that the same procedure takes place in the practical and in the speculative reason: for each proceeds from principles to conclusions, as stated above (De Lib. Arb. i, 6). Accordingly we conclude that just as, in the speculative reason, from naturally known indemonstrable principles, we draw the conclusions of the various sciences, the knowledge of which is not imparted to us by nature, but acquired by the efforts of reason, so too it is from the precepts of the natural law, as from general and indemonstrable principles, that the human reason needs to proceed to the more particular determination of certain matters. These particular determinations, devised by human reason, are called human laws, provided the other essential conditions of law be observed, as stated above (Q[90], AA[2],3,4). Wherefore Tully says in his Rhetoric (De Invent. Rhet. ii) that "justice has its source in nature; thence certain things came into custom by reason of their utility; afterwards these things which emanated from nature and were approved by custom, were sanctioned by fear and reverence for the law."
Reply to Objection 1: The human reason cannot have a full participation of the dictate of the Divine Reason, but according to its own mode, and imperfectly. Consequently, as on the part of the speculative reason, by a natural participation of Divine Wisdom, there is in us the knowledge of certain general principles, but not proper knowledge of each single truth, such as that contained in the Divine Wisdom; so too, on the part of the practical reason, man has a natural participation of the eternal law, according to certain general principles, but not as regards the particular determinations of individual cases, which are, however, contained in the eternal law. Hence the need for human reason to proceed further to sanction them by law.
Reply to Objection 2: Human reason is not, of itself, the rule of things: but the principles impressed on it by nature, are general rules and measures of all things relating to human conduct, whereof the natural reason is the rule and measure, although it is not the measure of things that are from nature.
Reply to Objection 3: The practical reason is concerned with practical matters, which are singular and contingent: but not with necessary things, with which the speculative reason is concerned. Wherefore human laws cannot have that inerrancy that belongs to the demonstrated conclusions of sciences. Nor is it necessary for every measure to be altogether unerring and certain, but according as it is possible in its own particular genus.
Whether there was any need for a Divine law?
Objection 1: It would seem that there was no need for a Divine law. Because, as stated above (A[2]), the natural law is a participation in us of the eternal law. But the eternal law is a Divine law, as stated above (A[1]). Therefore there was no need for a Divine law in addition to the natural law, and human laws derived therefrom.
Objection 2: Further, it is written ( Ecclus. 15:14 ) that "God left man in the hand of his own counsel." Now counsel is an act of reason, as stated above (Q[14], A[1]). Therefore man was left to the direction of his reason. But a dictate of human reason is a human law as stated above (A[3]). Therefore there is no need for man to be governed also by a Divine law.
Objection 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 inproportionate 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.
Reply to Objection 1: By the natural law the eternal law is participated proportionately to the capacity of human nature. But to his supernatural end man needs to be directed in a yet higher way. Hence the additional law given by God, whereby man shares more perfectly in the eternal law.
Reply to Objection 2: Counsel is a kind of inquiry: hence it must proceed from some principles. Nor is it enough for it to proceed from principles imparted by nature, which are the precepts of the natural law, for the reasons given above: but there is need for certain additional principles, namely, the precepts of the Divine law.
Reply to Objection 3: Irrational creatures are not ordained to an end higher than that which is proportionate to their natural powers: consequently the comparison fails.
Whether there is but one Divine law?
Objection 1: It would seem that there is but one Divine law. Because, where there is one king in one kingdom there is but one law. Now the whole of mankind is compared to God as to one king, according to Ps. 46:8 : "God is the King of all the earth." Therefore there is but one Divine law.