Question 90. The essence of law
Article 1. Whether law is something pertaining to reason?

I answer that, Law is a rule and measure of acts, whereby man is induced to act or is restrained from acting: for "lex" [law] is derived from "ligare" [to bind], because it binds one to act. Now the rule and measure of human acts is the reason, which is the first principle of human acts, as is evident from what has been stated above (1, 1, ad 3); since it belongs to the reason to direct to the end, which is the first principle in all matters of action, according to the Philosopher (Phys. ii). Now that which is the principle in any genus, is the rule and measure of that genus: for instance, unity in the genus of numbers, and the first movement in the genus of movements. Consequently it follows that law is something pertaining to reason. 

Article 2. Whether the law is always something directed to the common good?

I answer that, As stated above (Article 1), the law belongs to that which is a principle of human acts, because it is their rule and measure. Now as reason is a principle of human acts, so in reason itself there is something which is the principle in respect of all the rest: wherefore to this principle chiefly and mainly law must needs be referred. Now the first principle in practical matters, which are the object of the practical reason, is the last end: and the last end of human life is bliss or happiness, as stated above (2, 7; 3, 1). Consequently the law must needs regard principally the relationship to happiness. Moreover, since every part is ordained to the whole, as imperfect to perfect; and since one man is a part of the perfect community, the law must needs regard properly the relationship to universal happiness. Wherefore the Philosopher, in the above definition of legal matters mentions both happiness and the body politic: for he says (Ethic. v, 1) that we call those legal matters "just, which are adapted to produce and preserve happiness and its parts for the body politic": since the state is a perfect community, as he says in Polit. i, 1. 

Article 3. Whether the reason of any man is competent to make laws?

I answer that, A law, properly speaking, regards first and foremost the order to the common good. Now to order anything to the common good, belongs either to the whole people, or to someone who is the viceregent of the whole people. And therefore the making of a law belongs either to the whole people or to a public personage who has care of the whole people: since in all other matters the directing of anything to the end concerns him to whom the end belongs. 

Article 4. Whether promulgation is essential to a law?

I answer that, As stated above (Article 1), a law is imposed on others by way of a rule and measure. Now a rule or measure is imposed by being applied to those who are to be ruled and measured by it. Wherefore, in order that a law obtain the binding force which is proper to a law, it must needs be applied to the men who have to be ruled by it. Such application is made by its being notified to them by promulgation. Wherefore promulgation is necessary for the law to obtain its force.

Thus from the four preceding articles, the definition of law may be gathered; and it is nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated. 


Question 91. The various kinds of law
Article 1. Whether there is an eternal law?

On the contrary, Augustine says (De Lib. Arb. i, 6): "That Law which is the Supreme Reason cannot be understood to be otherwise than unchangeable and eternal."

I answer that, As stated above (90, 1, ad 2; A3,4), a law is nothing else but a dictate of practical reason emanating from the ruler who governs a perfect community. Now it is evident, granted that the world is ruled by Divine Providence, as was stated in the I, 22, A1,2, that the whole community of the universe is governed by Divine Reason. Wherefore the very Idea of the government of things in God the Ruler of the universe, has the nature of a law. And since the Divine Reason's conception of things is not subject to time but is eternal, according to Proverbs 8:23, therefore it is that this kind of law must be called eternal.


Article 2. Whether there is in us a natural law?


I answer that, As stated above (90, 1, ad 1), law, being a rule and measure, can be in a person in two ways: in one way, as in him that rules and measures; in another way, as in that which is ruled and measured, since a thing is ruled and measured, in so far as it partakes of the rule or measure. Wherefore, since all things subject to Divine providence are ruled and measured by the eternal law, it is evident that all things partake somewhat of the eternal law, in so far as, namely, from its being imprinted on them, they derive their respective inclinations to their proper acts and ends. It is therefore evident that the natural law is nothing else than the rational creature's participation of the eternal law. 


Article 3. Whether there is a human law?


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 (90, 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 (90, A2,3,4). 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."


Article 4. Whether there was any need for a Divine law?


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 (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.

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.