Summa Theologiae Moralis, vol. I: De Principiis

S. Th. I-II, q. 94, a. 2 and 3 ; q. 100, a. 1.

250. Natural law is one, containing several precepts. — Natural law, although with respect to the matter about which it is concerned it contains several precepts, is nevertheless in itself one, both because it tends to one ultimate end, and because all those precepts are founded on one first universal principle : Good is to be done, evil is to be avoided.

It is proved : Human reason, both speculative and practical, is essentially deductive. But just as being is the first thing known by the intellect, so good is the first thing known by practical reason. Whence just as the first speculative principle is founded on the notion of being : It is impossible for the same thing to be and not to be, so the first practical principle is founded on the notion of good : Good is to be done, evil avoided ; and therefore reason prescribes or forbids nothing except what it naturally apprehends as good or evil¹.

251. Order and division of the precepts. — Natural law in a general way comprises all those things which so befit or are repugnant to rational nature that for the preservation of order they must necessarily be prescribed or forbidden by the very Creator of nature²; whence there pertain to natural law all the precepts to which by the nature of the thing and independently of any positive law we are obliged. Among which there is a manifold order :

A. By reason of matter, in a twofold way : a) according to the threefold natural inclination of man to the good, whether of natural being, i.e. the preservation of existence, or of animal life, e.g. the procreation of offspring, or of rational life proper to himself.

For good has the character of an end, whence man has as goods all those things to which he has a natural inclination. But there is in man a threefold inclination : 1) to the good according to the nature which he has in common with all substances, which all seek the preservation of their being ; 2) to the good according to the nature which he has in common with animals, such as procreation and education of offspring ; 3) to the good according to the nature of reason, which are proper to him, e.g. that he worship God and live in society.

Hence the precepts are distinguished according to the nature of substance (abstinence, fortitude), according to the nature of animal (chastity, prudence), and according to the nature of man (religion, justice) ;

b) according to the threefold natural relation or order of man : 1) to God as to his first principle and ultimate end ; — 2) to himself : for he is made up of two parts, of which the higher must rule the lower ; — 3) to his neighbor : born social, man is destined for conjugal, domestic, and political society. Hence the precepts are distinguished into those toward God, toward himself, and toward his neighbor, according as man is considered as a created, rational, or social being.

The order to God is the order of subjection ; the order to one's neighbor is the order of equality and proximity, by which his person in himself or in his goods is forbidden to be violated, and he himself is to be loved after our own likeness ; the order of man to himself is the order of conformity of himself and of his faculties (of which he is not the master) to reason and to rational nature and its proper end, and of subordination to the human species and the common good.

B. Formally, on the part of the manner in which they are contained in law, or by reason of the knowledge and logical nexus between the principles of natural law :

1. some are general, most universal, most common, indeterminate, evident in themselves, known to all from the mere understanding of the terms, e.g. Good is to be done, evil is to be avoided ; What you do not wish to be done to you, do not do to another ; One is to live according to reason ; The Creator is to be obeyed ; To each his own is to be rendered ; The Supreme Good is to be loved ; Injury is to be done to no one ; Live conformably to your nature as a rational, created, social being, etc.

2. others are more proper and determinate, particular, as conclusions deduced from the first, and these are

a) either immediately, proximately and evidently deduced from the first, which are the precepts of the decalogue ; or

b) more remote, which are not so evidently but mediately and by many consequences deduced, e.g. that usury is unjust, that lying is always illicit, that injury inflicted is not to be vindicated by one's own authority, that things found are to be restored.

C. By reason of force and efficacy, or necessity for the end, and the ontological and real nexus :

1. some precepts are primarily intended, which nature altogether requires, because they are altogether necessary for the end which nature directly and principally intends : thus matrimony is altogether necessary for the due procreation and education of offspring ;

2. others are secondarily intended, derived from the first, to which nature much inclines, which are either necessary, not indeed in order that the primary end be obtained, but in order that it be obtained becomingly, suitably, more easily, or are necessary for a secondary end which nature does not directly but concomitantly and subordinately to another intend : thus the unity of matrimony befits the education of offspring and is necessary for family peace.

Briefly : some precepts are about things necessary without which the moral order is impossible ; others, about very useful things, which are not strictly necessary, so that the order without them, although less perfectly, can subsist³.

Hence with relation to the altogether ultimate and primary end of natural law, which is God and the love of God above all things, are distinguished

a) first precepts, which strictly necessary for the love of God prescribe, or forbid those things that directly hinder it and turn away from it ; of which

1. some immediately state the necessary order, or repugnance, to the ultimate end, God : these are the precepts of the first table ;

2. others immediately state the necessary order, or repugnance, to the love of ourselves or of our neighbor, yet mediately to the ultimate end, inasmuch as they are means by the nature of the thing necessary for it : the love of ourselves indeed, because it is a necessary predisposition to the love of God and of our neighbor, since we can tend only to our own good, nor love anyone except by imitating the love of ourselves, and because our life is a necessary condition for the attainment of the end ; the love of our neighbor, because society is a medium altogether necessary by the nature of the thing for men to obtain the end, whence those that are necessary for society without which it cannot subsist pertain here : these are the precepts of the second table.

All of which are prescribed or forbidden gravely, as altogether necessary for the ultimate end or altogether opposed to it ;

b) second precepts derived from the first, which prescribe those things that are necessary in order that the end be obtained becomingly, better, more easily, and forbid those things that retard it or cannot be referred to it, such as are 1) those that include a certain small irreverence toward God, and only fall short of the due manner of worship ; 2) those that somewhat detract from justice or charity ; or 3) those that only lack necessity or utility. These are prescribed or forbidden lightly, because they are not strictly necessary for the ultimate end nor turn away from it⁴.

From the foregoing it is clear that the primary or secondary in cognition and the primary or secondary in necessity and intention of nature are not the same precepts ; for many secondary in cognition which are deduced from the first principles by conclusions (even remote) are primarily intended by nature, as the prohibition of homicide, adultery, theft ; hence one must well observe in what sense those terms are used by authors.

252. The object of natural law are the acts of the virtues. — The acts of the virtues, according to genus, inasmuch as they are virtuous, all are of natural law : for this comprises all things to which man is inclined according to nature. But each thing is inclined to the operation befitting its form ; now the form of man is the rational soul. Whence man is naturally inclined to operate according to reason, which is to act according to virtue. — If however virtuous acts are considered in themselves, inasmuch as they are such and such, according to species, not all are of natural law, because many are of counsel, or of those things to which nature does not incline, but which men have discovered by the inquiry of reason as useful for living well.

253. The subject of natural law. — Because natural law is implanted with rational nature and essentially conjoined to it, hence all rational creatures are subject to it and bound by it, not only adults (among whom the blessed and the damned), but also children and those permanently demented : these however not perfectly and in the second act, because they do not have the use of reason and knowledge of the law, but imperfectly and in the first act, because they have the potency for these. Hence

1. although in these there is no formal sin because the use of reason and of liberty is lacking, they nevertheless sin materially, by acting against the law of nature inherently in them habitually ;

2. the same is to be said of those sleeping, drunk, and others who here and now are ignorant of the law, unless however they have culpably placed the cause of sin, in which case they also sin formally ;

3. those who induce the aforesaid to act against natural law sin formally : for they are the moral or even physical cause of these sins, and therefore by their will they participate in and embrace them : for the action of the instrument is rightly imputed to the principal agent. Add that they are often the cause of evil custom in those, and of scandal in those standing by.