Section II. On Natural Law

I. Notion of natural law. "Natural law," says St. Thomas, "is nothing other than a participation of the eternal law in the rational creature" (I-II, q. 91, a. 2). But a natural participation is understood, so that it may be distinguished from the participation of eternal law which is made by means of divine revelation. Now the said natural participation consists in human reason itself, insofar as it naturally knows what must be done or avoided, and intimates or announces this as something to be done or avoided. It is said "naturally," that is, by means of nature itself or by means of the inclination of nature; these things will be clearer from what is to be said.

Corollary. Natural law differs from eternal law insofar as the latter is supreme, unparticipated, eternal, and in God; but natural law is lower, participated, temporal, and in man.

II. Existence of natural law. This is clear: 1. from Sacred Scripture. St. Paul¹ declares: "For when the gentiles, who do not have the law, do by nature those things that are of the law, such persons, not having the law, are a law to themselves. They show the work of the law written in their hearts, their conscience bearing witness to them, and their thoughts among themselves accusing or also defending them, on the day when God will judge the hidden things of human beings, according to my Gospel, through Jesus Christ." 2. Leo XIII teaches thus: "Such is the chief of all laws, the natural law, which is written and engraved in the souls of individual human beings, because it is human reason itself commanding them to do rightly and forbidding them to sin."² 3. Natural law, as we have said, is a participation of eternal law, which consists in human reason, insofar as this naturally knows what must be done or avoided and intimates or announces it as something to be done or avoided. But such a participation of eternal law is found in man. Human reason is in him, as is clear. But it naturally knows what must be done or avoided, namely by means of the inclination of nature; for from the fact that nature is inclined toward something, reason apprehends that as agreeing with nature or as a good of nature, and therefore as to be pursued; but from the fact that nature turns away from something, reason apprehends that as repugnant to nature or as an evil of nature, and therefore as to be avoided. Once the said apprehension of a good to be pursued or an evil to be avoided is posited, reason naturally intimates or announces the pursuit or flight as to be carried out in deed. And thus it has been proved that all those things which are comprehended in the notion of natural law are found in man; in other words, that natural law exists. 4. There are added the testimony of conscience and the common persuasion of human beings.

¹ Rom. 2:14-16.

² Encyclical Libertas, June 20, 1888.

III. Object of natural law is that which man naturally apprehends as good or evil; naturally, that is, as we said above, by means of nature itself or the inclination of nature. And because the inclinations of nature are distinguished in three ways, namely according to the nature of substance, according to the nature of animal, and according to the nature of reason, therefore the objects themselves of natural law can also be distinguished in three ways.

"For there is first in man," says St. Thomas, "an inclination to good according to the nature in which he shares with all substances, namely according as any substance desires the conservation of its own being according to its nature; and according to this inclination there pertain to natural law those things by which the life of man is conserved and the contrary is impeded. Second, there is in man an inclination to certain more special things according to the nature in which he shares with other animals; and according to this those things are said to be of natural law which nature has taught all animals, such as the union of male and female, the education of offspring, and similar things. In the third way, there is in man an inclination to good according to the nature of reason, which is proper to him, as man has a natural inclination to this: that he know the truth about God, and that he live in society. And according to this, those things which pertain to an inclination of this kind belong to natural law, for instance that man avoid ignorance, that he not offend others with whom he ought to live, and the other things of this sort which pertain to this" (St. Thomas, I-II, q. 94, a. 2).

IV. Subject of natural law is all human beings, because the light of reason is infused into all, by which, at least of itself and in potency, they naturally know what must be done and what must be avoided. Hence all, even infants and the perpetually insane, are bound by the said law objectively or in first act, as it is said; but in order that someone be bound by it subjectively or in second act, knowledge of it is required.

V. Knowability of natural law. Concerning this, it must first be seen how far the objects, and consequently the precepts, of natural law are distinguished when their knowability is considered; then how far ignorance can in fact have place concerning them.

1. The objects, and consequently the precepts, of natural law, by reason of knowability, are distinguished into primary, secondary, and tertiary. Primary precepts are those which become known at first glance, as: good is to be done; evil is to be avoided; order is to be preserved. Secondary precepts are those which are immediately and easily deduced from the primary ones. To these belong the precepts of the Decalogue, except the circumstance of the third precept by which the day to be sanctified is determined. Tertiary precepts are those which are deduced mediately and with more difficulty from the primary or secondary ones, such as, for example, precepts concerning certain contracts to be avoided (St. Thomas, I-II, q. 94, aa. 4 and 6; q. 100, a. 1; St. Alphonsus, I, 171; Homo Apostolicus, tr. 1, n. 5).

2. Ignorance with respect to the precepts of natural law can have place in a human being of sound mind as follows:

a) Ignorance cannot be given with respect to primary precepts, because "these stand to practical reason as the first principles of demonstrations stand to speculative reason" (St. Thomas, I-II, q. 94, a. 2). Moreover, first principles become known to each person (St. Alphonsus, I, 170).

b) Generally ignorance is not given with respect to secondary precepts considered in themselves. This is clear both from the notion of them handed down and from experience. I have said "generally" and "considered in themselves," because sometimes ignorance is present "in rude persons and for a brief time, or... with some apparently honorable circumstance added" (St. Alphonsus, Homo Apostolicus, tr. 1, n. 5; St. Thomas, I-II, q. 94, a. 6), for example if there is question of a lie for saving the life of an innocent person.

c) Concerning tertiary precepts, ignorance is more easily had, as is deduced from their notion and from daily experience (St. Thomas, I-II, q. 100, a. 1; St. Alphonsus, I, 171).

VI. Mutability of natural law. First it must be noted that change is taken in two ways, improperly and properly. Change improperly so called is had: 1. when something is added to natural law through positive law; 2. when the matter which was the object of the law is so changed that it ceases to be the object of the law, as when God, as supreme Lord of all goods, transferred the spoils of the Egyptians into the dominion of the Hebrews.

Change properly so called is had when, while the matter of the law persists, the law ceases, or at least does not oblige someone.

Change properly so called is distinguished in two ways, from within and from without. Change from within has place when the law becomes useless or harmful, or the time for which the law was established has elapsed. Change from without has place when a superior totally or partially removes the law or dispenses in it. Having noted these things, I say:

1. Change improperly so called, if it is understood in the sense that something is added to natural law, manifestly can have place. "For many things useful to human life have been added over and above natural law both through divine law and also through human laws" (St. Thomas, I-II, q. 94, a. 5).

2. Change improperly so called, which consists in the change of matter, can be had in those things which are constituted as the object of law dependently on some act, whether of God or of man, for example dependently on the act by which someone has made some good his own, or has taken some person to himself as wife, etc. The reason is that at least God can invalidate the said act or deprive it of its effect (cf. Corollary II). Change improperly so called cannot have place in those things which through themselves, or independently of some act, constitute the object of law, whether prescriptive or prohibitive, such as love and hatred of God, truthful speech and lying, etc. The reason is that the essence itself of the thing cannot be changed; hence the aforesaid things necessarily remain objects of law.

3. Change properly so called and from within is impossible; for natural law is founded in human nature itself, and therefore, if this nature persists unchanged, it cannot become useless or harmful to that nature. But human nature is immutable.

4. Change properly so called and from without, for example by dispensation, likewise seems impossible. For the goodness and malice of the acts which natural law prescribes or forbids do not depend on the free will of God. But what is evil, for example, independently of the free will of God, cannot be permitted by him. Thus teach St. Thomas (I-II, q. 100, a. 8) and authors much more commonly, against Scotus and his disciples, who judge that God can dispense in the precepts of the second table of the Decalogue, as it is called, except for the precept concerning lying.

Corollary I. No change of natural law has place where the law itself does not extend itself to some case. Thus, for example, natural law forbids killing a human being; nevertheless, with the things that must be posited being posited, it is permitted to kill an unjust aggressor. Natural law forbids marriage between brothers and sisters; nevertheless, marriage between the sons and daughters of Adam was lawful on account of the necessity of propagating the human race. From these and other examples it is clear that natural laws, for example the law of not killing, are rather often not adequately expressed by the formulas customarily employed by us.

Corollary II. Several cases occur in Sacred Scripture in which, according to St. Thomas and others, change improperly so called had place. Namely, God, as the supreme Lord of all persons and things, can grant and take away rights. "And therefore when the children of Israel took the spoils of the Egyptians by the command of God, it was not theft, because this was owed to them by the sentence of God. Similarly, when Abraham consented to kill his son, he did not consent to homicide, because it was owed that he be killed through the command of God, who is Lord of life and death; for he himself is the one who inflicts the penalty of death on all human beings, just and unjust, for the sin of the first parent. If a human being is the executor of this sentence by divine authority, he will not be a murderer, just as God is not. And likewise Hosea, approaching a fornicating wife or an adulterous woman, did not commit adultery or fornication, because he approached her who was his according to the divine command, from him who is the author of the institution of marriage" (St. Thomas, I-II, q. 100, a. 8).¹ By the same reason, God, on account of a special reason, can grant divorce even with respect to the bond, polygamy, by dividing the power over the body of the man among several women, etc. Whether and to what extent such power has also been entrusted to the Church will be explained in its own place.

Corollary III. Dispensation in vows rests on the fact that the competent authority remits in the name of God a debt owed to God.

¹ Others explain these and similar cases otherwise, or even have recourse to the said cases to prove that change of natural law properly so called, or dispensation, is possible (cf. Billuart, De legibus, diss. 2, a. 4).

VII. Source of obligation. Natural law does not oblige independently of God. This is established against the supporters of independent morality, who indeed admit natural obligation, but derive it not from God but from elsewhere, for example from human reason, Kantianism or autonomy of reason.¹ The proposition is proved from the fact that obligation, that is, an intrinsic and moral restriction of physical liberty, cannot come except from a Superior. But man's reason is not superior to man himself; nor, when authority granted by God to someone has been set aside, is one person subject to another. Of themselves, all are equal, and God alone is man's superior. Hence the third proposition of the Syllabus was condemned: "Human reason, with no regard at all had for God, is the only arbiter of the true and the false, of good and evil; it is a law to itself."² (St. Thomas, I-II, q. 93, a. 3).

¹ Others derive the obligation of placing or omitting an action from the fact that it is useful or harmful. Thus utilitarianism, the philosophy of positivism, and Darwinian ethics. Vasquez (In I-II, disp. 150, c. 3), Grotius (De iure belli ac pacis, Proleg. 11), and others judge that some obligation arises antecedently to divine ordinance from the conformity of actions with human nature; by approving and willing this obligation, God induces a new obligation.

² Cf. Encyclical Libertas, June 20, 1888.