From James Rachels, "The Right Thing to Do: Basic Readings in Moral Philosophy", 2nd ed. (New York: McGraw Hill, 1999)
From Saint Thomas Aquinas, Summa Theologica, First Part of the Second Part. In Basic Writings of St. Thomas Aquinas, edited by Anton C. Pegis, vol. 2 (New York: Random House, 1945), pp. 749-753, 776-778.
Question 91
Second Article: Whether There Is in Us a Natural Law? ... The Gloss on Rom. 2:14 (When the Gentiles, who have not the law do by nature those things that are of the law) comments as follows: Although they have no written law, yet they have the natural law, whereby each one knows, and is conscious of, what is good and what is evil. ... 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. Therefore, since all things subject to divine providence are ruled and measured by the eternal law . . . it is evident that all things partake in some way in 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. Now among all others, the rational creature is subject to divine providence in a more excellent way, in so far as it itself partakes of a share of providence, by being provident both for itself and for others. Therefore 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. Hence the Psalmist, after saying (Ps. 4:6): Offer up the sacrifice of justice, as though someone asked what the works of justice are, adds: Many say, Who showeth us good things? in answer to which question he says: The light of Thy countenance, O Lord, is signed upon us. He thus implies that 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. It is therefore evident that the natural law is nothing else than the rational creature's participation of the eternal law.
Third Article: Whether There Is a Human Law? ... Augustine distinguishes two kinds of law, the one eternal, the other temporal, which he calls human.... As we have stated above, a law is a dictate of the practical reason.... 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 that from the precepts of the natural law, as from common and indemonstrable principles, 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 that the other essential conditions of law be observed.... Therefore Tully says in his Rhetoric 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. ... just as on the part of the speculative reason, by a natural participation of divine wisdom, there is in us the knowledge of certain common principles, but not a 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 common 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.
Fourth Article: Whether There Was Any Need for a Divine Law? ... 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. Now if man were ordained to no other end than that which is proportionate to his natural ability, there would be no need for man to have any further direction, on the part of his reason, in addition to the natural law and humanly devised law which is derived from it. But since man is ordained to an end of eternal happiness which exceeds man's natural ability, . . . therefore it was necessary that, in addition to the natural and the human law, man should be directed to his end by a law given by God.
Secondly, because, by reason 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 are observable; and yet for the perfection of virtue it is necessary for man to conduct himself rightly 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, 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 living. 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....
Question 94
Third Article: Whether All the Acts of the Virtues Are Prescribed by the Natural Law? ... We may speak of virtuous acts in two ways: first, in so far as they are virtuous; secondly, as such and such acts considered in their proper species. If, then, we are speaking of the acts of the virtues in so far as they are virtuous, thus all virtuous acts belong to the natural law. For it has been stated that to the natural law belongs everything to which a man is inclined according to his nature. Now each thing is inclined naturally to an operation that is suitable to it according to its form: e.g., fire is inclined to give heat. Therefore, since the rational soul is the proper form of man, there is in every man a natural inclination to act according to reason; and this is to act according to virtue. Consequently, considered thus, all the acts of the virtues are prescribed by the natural law, since each one's reason naturally dictates to him to act virtuously. But if we speak of virtuous acts, considered in themselves, i.e., in their proper species, thus not all virtuous acts are prescribed by the natural law. For many things are done virtuously, to which nature does not primarily incline, but which, through the inquiry of reason, have been found by men to be conducive to well-living....
Temperance is about the natural concupiscences of food, drink and sexual matters, which are indeed ordained to the common good of nature, just as other matters of law are ordained to the moral common good....
By human nature we may mean either that which is proper to man, and in this sense all sins, as being against reason, are also against nature, as Damascene states; or we may mean that nature which is common to man and other animals, and in this sense, certain special sins are said to be against nature: e.g., contrary to sexual intercourse, which is natural to all animals, is unisexual lust, which has received the special name of the unnatural crime....
Fourth Article: Whether the Natural Law Is the Same in All Men? ... As we have stated above, to the natural law belong those things to which a man is inclined naturally; and among these it is proper to man to be inclined to act according to reason. Now it belongs to the reason to proceed from what is common to what is proper, as is stated in Physics i. The speculative reason, however, is differently situated, in this matter, from the practical reason. For, since the speculative reason is concerned chiefly with necessary things, which cannot be otherwise than they are, its proper conclusions, like the universal principles, contain the truth without fail. The practical reason, on the other hand, is concerned with contingent matters, which is the domain of human actions: and, consequently, although there is necessity in the common principles, the more we descend towards the particular, the more frequently we encounter defects. Accordingly, then, in speculative matters truth is the same in all men, both as to principles and as to conclusions; although the truth is not known to all as regards the conclusions, but only as regards the principles which are called common notions. But in matters of action, truth or practical rectitude is not the same for all as to what is particular, but only as to the common principles; and where there is the same rectitude in relation to particulars, it is not equally known to all.
It is therefore evident that, as regards the common principles whether of speculative or of practical reason, truth or rectitude is the same for all, and is equally known by all. But as to the proper conclusions of the speculative reason, the truth is the same for all, but it is not equally known to all. It is true for all that the three angles of a triangle are together equal to two right angles, although it is not known to all. But as to the proper conclusions of the practical reason, neither is the truth or rectitude the same for all, nor, where it is the same, is it equally known by all. Thus, it is right and true for all to act according to reason, and from this principle it follows, as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is true for the majority of cases. But it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance, if they are claimed for the purpose of fighting against one's country. And this principle will be found to fail the more, according as we descend further towards the particular, e.g., if one were to say that goods held in trust should be restored with such and such a guarantee, or in such and such a way; because the greater the number of conditions added, the greater the number of ways in which the principle may fail, so that it be not right to restore or not to restore.
Consequently, we must say that the natural law, as to the first common principles, is the same for all, both as to rectitude and as to knowledge. But as to certain more particular aspects, which are conclusions, as it were, of those common principles, it is the same for all in the majority of cases, both as to rectitude and as to knowledge; and yet in some few cases it may fail, both as to rectitude, by reason of certain obstacles (just as natures subject to generation and corruption fail in some few cases because of some obstacle), and as to knowledge, since in some the reason is perverted by passion, or evil habit, or an evil disposition of nature. Thus at one time theft, although it is expressly contrary to the natural law, was not considered wrong among the Germans, as Julius Caesar relates.