Commentaria in Summam contra Gentiles, Liber III

Because someone could think, indeed some have held, that those things which are commanded by divine law are right solely on the ground that they have been posited by law, St. Thomas wishes to show that things of this sort not only have rectitude because they have been posited by law, that is, established and ordained by law, but also according to nature.

I. And he argues first as follows. By the precepts of divine law the mind of man is ordered under God; and all other things that are in man, under reason. Therefore, those things which are commanded by divine law are naturally right in themselves. — The consequence is proved. Because the natural order requires that lower things be subject to higher things.

Secondly. Men, by divine providence, receive the natural faculty of judgment of reason as the principle of their proper operations. Therefore, there are some operations naturally suitable to man, which are right in themselves. — The consequence is proved. Because natural principles are ordered to those things that exist naturally.

Thirdly. The nature of men is determinate. Therefore, there must be some operations in themselves suitable to man. — The consequence is proved. Because of whatever things there is a determinate nature, there must be determinate operations that are suitable to that nature; since the operation of each thing follows its nature.

It should be noted that this argument takes as manifest that those things which are suitable to a nature of themselves are of themselves good and right; because nature does not incline to evil, but rather to good. This is what is meant when something is said to be suitable to nature. And therefore, since through this argument it has been concluded that man has some operations in themselves suitable to him, it is left sufficiently manifest that some operations of man are good according to his nature.

II. Fourthly. It is natural to man that he is a social animal. Therefore, it is also natural that he preserve to each one what is his own, and that he abstain from injuries. Therefore, etc. — The antecedent is proved. Because one man is not sufficient for all the things that are necessary for human life. — But the consequence is proved. Because to whatever something is natural, that without which it cannot be preserved is also natural; since nature is not deficient in necessary things. But human society cannot be preserved unless what is his own is preserved to each one, and injuries are abstained from.

Fifthly. Every man naturally has it that he uses lower things for the necessity of his life. But in the use of things there is a certain determinate measure suitable to human life, which, if it be neglected, causes harm to man; as is apparent in the inordinate consumption of food. Therefore, there are some acts naturally suitable to man.

It should be noted that this argument is grounded upon this manifest principle: That in man is naturally right which does not exceed the determinate measure instituted by nature. Because such a thing is in accordance with reason; but everything that is in accordance with reason is right.

III. Sixthly. According to the natural order, the body of man exists for the sake of the soul, and the lower powers of the soul for the sake of reason. This is made clear by the likeness of matter and of an instrument. Therefore, it is naturally right that the body and the lower powers of the soul be so cared for by man that from this the act of reason and its good are not at all impeded. But if it should turn out otherwise, it will be naturally a sin. — The consequence is proved. Because from that which is ordered to another, help is said¹ to come to the thing, not an impediment.

From this argument St. Thomas infers that drunkenness, gluttony, and inordinate sexual acts, through which the act of reason is impeded; and being subjected to passions, which do not allow the judgment of reason to be free: are naturally evil.

This argument has as its foundation that that is naturally right which occurs according to the order and institution of nature; on the contrary, however, that is naturally evil by which the order of nature is perverted or impeded.

Seventhly. Man is naturally ordered to God as to an end. Therefore, those things by which man is led to the knowledge and love of God are naturally right; but those which are the opposite are naturally evil. — The consequence is proved. Because to each thing are naturally suitable those things by which it tends toward its natural end; but those which are the opposite are naturally unsuitable to it.

IV. From the foregoing St. Thomas infers that good and evil in human acts are not only according to the positing of law, but also according to the natural order. Which he confirms by the authority of the Psalm.

By this, moreover, is excluded the error of those who say that just and right things are posited solely according to law.²

It should be noted that St. Thomas, in several of the aforesaid arguments, concluded only this: that there are some things in human acts that are right according to nature; because this was sufficient for him to exclude the position of those who posited that the things commanded by divine law are not right except because they have been posited by law, for the reason that they said nothing is right and just except according to what has been posited and instituted by law. For by the fact that it is shown that some things are right according to nature, it is evident that the foundation of their position has been destroyed.