The following excerpts surmising Thomas Aquinas on Law and the Common Good are taken from How American Law Lost God by Father John Luke McDermott J.D, 2012.

Aquinas on Natural Law

2.10 Aquinas and Natural Law Precepts St. Thomas Aquinas distinguished divine law revelation from natural law – that is, moral imperatives that man can perceive with his reason in a n order universe created by God. (New Catholic Encyclopedia 202). Aquinas described the principal precepts of natural law: existence; self-preservation; the family; and education, society and (natural) religion. (Aquinas, Summa Theologiae, 1a 2ae, 94.2) Aquinas held that human laws contrary to natural law are null and void. (Ibid., 96.2). 

Aquinas on the Common Good

5.1 Common Good True good is the common good regulated according to divine justice. (Aquinas, Summa Theologiae, Pt. I-II Q. 92 Art. 1.) St. Thomas Aquinas identifies the common good with the virtuous life of the citizen. Aquinas states:

It is impossible that a man be good, unless he be well ordered as to the common good, nor can the whole be well ordered unless its parts be proportioned to it. Consequently, the common good of the State cannot flourish unless the citizens be virtuous, at least those whose business it is to govern. But it is enough for the good of the community that the other citizens be so far virtuous that they obey the commands of their rulers. (Ibid.)

The private good can be prior to the common good if the private good is not in the same genus as the common good. Virginity has priority over carnal fecundity. In every case in which the good of the individual and the common good are in the same genus, the common good is prior, and the search for the common good is indispensable for achieving man’s own good, the virtuous life. (Ibid.)

Aquinas on Law

6.1 Four Types of Law. Law is an ordinance of reason for the common good, made by him who has care of the community and promulgated. (Aquinas, Summa Theologiae, Pt. II-II, Q. 60, Art. 4) Law has four types: Eternal Law, Natural Law, Human Law and Divine Law. (Aquinas, Summa Theologiae, Pt. I-II, Q. 91, Art. 1-5)

6.1.1. Eternal Law. The world is ruled by Divine Providence; the whole community of the universe is governed by Divine Reason. The very idea of the government of things in God, the Ruler of the Universe, has the nature of a law. (Aquinas, Summa Theologiae, Pt. I-II, Q. 91, Art. 1) Since Divine Reason’s conception of things is not subject to time but is eternal, this kind of law must be called eternal. Id. Those things that are not in themselves, exist with God, inasmuch as they are foreknown and preordained by Him, according to Rom. 4:17, Who calls those things that are not, as those that are. The eternal concept of the divine law bears the character of an eternal law insofar as it is ordained by God to the government of things foreknown by Him. Promulgations are made by word of mouth or in writing; and in both ways the eternal law is promulgated: because both the Divine Word and the writing of the Book of Life are eternal. The law implies order to the end actively, insofar as it directs certain things to the end, but not passively; the law itself is not ordained to the end — except accidentally in a governor whose end is extrinsic to him, and to which end, his law must needs be ordained. But the end of the divine government is God Himself, and His law is not distinct from Himself. Wherefore, the eternal law is not ordained to another end.

6.1.2. Natural Law. All things subject to Divine Providence are ruled and measured by the eternal law. All things partake somewhat of the eternal law, insofar as namely, from its being imprinted on them, they derive their respective inclinations to their proper acts and ends (Aquinas, Summa Theologiae, Pt. I-II, Q. 91, Art. 2). The rational creature is subject to Divine Providence in the most excellent way, insofar as it partakes of a share of providence, by being provident both for itself and for others. Wherefore, it has a share of the Eternal Reason, whereby it has a natural inclination to its proper act and end: and this participation of the eternal law in the rational creature is called the natural law. (Ibid.) The light of natural reason, whereby we discern what is good and what is evil, which is the function of the natural law, is nothing else than an imprint on us of the divine light. (Ibid.). The natural law is nothing else than the rational creature’s participation in the eternal law. Id.

6.1.3. Human Law. A law is a dictate of the practical reason. The same procedure takes place in the practical and in the speculative reason: for each proceeds from principles to conclusions. (Aquinas, Summa Theologiae, Pt. I-II, Q. 91, Art. 3) 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. (Ibid.) On the part of the practical reason, man has a natural participation in an eternal law, according to certain general principles, but not as regards the particular determinations of individual cases, which are contained in the eternal law. Hence, the need for human reason to proceed further to sanction them by law.

6.1.4. Divine Law. Besides the natural and the human law, it is necessary for the directing of human conduct to have a divine law, for four reasons:

6.1.4.1. The End is Eternal Happiness. Because it is by law that man is directed how to perform his proper acts in view of his last end. Since man is ordained to an end of eternal happiness which is proportionate to man’s natural facility, it was necessary that besides the natural and the human law, man should be directed to this end by a law given by God.

6.1.4.2. Direction of Proper Acts. 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. (Aquinas, Summa Theologiae, Pt. I-II, Q. 91, Art. 4)

6.1.4.3. Divine Law for Interior Acts. 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. Human law could not sufficiently curb and direct interior acts; and it was necessary for this purpose that a divine law should supervene. (Ibid.)

6.1.4.4. Divine Law Forbids All Sins. 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. In order that no evil might remain unforbidden and unpunished, it was necessary for the divine law to supervene, whereby all sins are forbidden. (Ibid.)

6.2. Divine Law: Two Parts. Divine law is divided into Old and New. (Aquinas, Summa Theologiae, Pt. I-II, Q. 91, Art. 5) The perfection and imperfection of these two laws are to be taken in connection with the three conditions pertaining to law:

6.2.1. Law’s End: The Common Good. Law directs to the common good as to its end. Id. This good may be twofold. Firstly, it may be a sensible and earthly good such as directly ordained by the Old Law. Id. Secondly, the good may be an intelligible and heavenly good, such as that to which man is ordained by the New Law. Id. Augustine says (Contra Faust. IV) that promises of temporal goods are contained in the Old Testament, for which reason it is called “Old;” but the promise of eternal life belongs to the New Testament. Id.

6.2.2. Law Directs Acts. Law directs human acts according to the order of righteousness: wherein also the New Law surpasses the Old Law. Id. Matt. 5:20: “Unless your justice abound more than that of the Scribes and Pharisees, you shall not enter into the Kingdom of Heaven.” The Old Law restrains the hand, but the New Law controls the mind. Id.

6.2.3. Law Induces Obedience. Law induces men to observe its commandments. (Ibid.) This the Old Law did by the fear of punishment: but the New Law, by love, which is poured into our hearts by the grace of Christ, bestowed in the New Law, but foreshadowed in the Old. (Ibid.) “There is no other name… [than Christ] given to men, whereby we must be saved.” (Acts 4:12)

6.3. Effects of Law. Law is a dictate of reason in the ruler by whom his subjects are governed. The virtue of every subject consists in his being subjected to his ruler, as Aristotle says. (Aristotle, Polit. i.) Every law aims at being obeyed by those who are subject to it. Consequently, it is evident that the proper effect of law is to lead its subject to proper virtue: and since virtue is that which makes its subject good, it follows that the proper effect of law is to make those to whom it is given, good, either simply or in some particular respect. (Aquinas, Summa Theologiae, Pt. I-II Q 92 Art 1.) [Emphasis added.] For if the intention of the lawgiver is fixed on true good, which is the common good regulated according to divine justice, it follows that the effect of the law is to make men good simply. (Ibid.)

6.4. Natural Law Precepts. Things to which man has a natural inclination are naturally apprehended by reason as being good. (Aquinas, Summa Theologiae, Pt. I-II Q 92 Art 2.). The order of natural inclinations is the order of the precepts of the natural law. (Ibid.) Every substance has the inclination to preserve its own being. (Ibid.) Whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law. (Ibid.) Man’s right to life shall be considered, infra. Secondly, natural law contains what nature has taught to all animals: such as the generation and education of offspring. (Ibid.) Man’s natural right to generate and educate offspring shall be discussed, infra. (Ibid.) Thirdly, man has an inclination to good, according to the nature of his reason, which nature is proper to him: thus, man has a natural inclination to know the truth about God and to live in society. (Ibid.)