Summa contra Gentiles3.128 Chapter CXXVIII. How by the divine law man is directed to his neighbour

Accordingly, from what has been said, it is clear that man is led by the divine law to observe the order of reason in all things that may be of use to him. Now, of all things that may be useful to man, other men hold the first place, since man is by nature a social animal: for he needs many things that cannot be provided by one man alone. Therefore it behoves man to be instructed by divine law so as to behave towards other men according to the order of reason.

Again. The end of the divine law is that man may adhere to God. Now, in this, one man is assisted by another both in his knowledge and in his affections: because one man helps another to know the truth; and one urges another to good, and withdraws him from evil. Hence it is said (Prov. xxvii. 17): Iron sharpeneth iron, so a man sharpeneth the countenance of his friend; and (Eccles. iv. 9-12): It is better... that two should be together, than one: for they have the advantage of their society: if one fall he shall be supported by the other. Woe to him that is alone, for when he falleth, he hath none to lift him up. And if two lie together, they shall warm one another; how shall one alone be warned? And if a man prevail against one, two shall withstand him.. Therefore it behoved the divine law to direct the mutual relations between man and man.

Moreover. The divine law is a rule of divine providence for the governance of men. Now, it belongs to divine providence to keep all things subject to it within the bounds of right order: so that, to wit, each thing be in its place and degree. Accordingly, the divine law directs men to one another in such wise that each one remains in his own order: which is for men to be at peace with one another, for peace among men is nothing else but rightly ordered harmony, as Augustine says.

Besides. Whenever a number of things are subordinate to one, they ought to be harmoniously ordered to one another: else they would hinder one another in the prosecution of the common end: thus, an army is harmoniously ordered to victory which is the end of the commander-in-chief. Now, every man is ordered to God by the divine law. Therefore it behoved the divine law to establish an ordered harmony, which is peace, among men, lest they be a hindrance to one another.

Hence it is said in the psalm: Who hath placed peace in thy borders: and our Lord said (Jo. xvi. 33): These things I have spoken to you, that in me you may have peace.

Now ordered harmony is observed among men when to every one is given his due: and this belongs to justice. Wherefore it is said (Isa. xxxii. 17): The work of justice shall be peace. Therefore it behoved the divine law to give precepts of justice, that every one might give others their due, and refrain from doing them wrong.

Among men our greatest debt is to our parents. Wherefore the first of the legal precepts (Exod. xx. 12-17) that order us to our neighbour is: Honour thy father and thy mother, whereby we are to understand that each one is commanded to give their due both to his parents and to others, according to Rom. xiii. 7: Render to all men their dues.—The next place is given to the precepts that forbid wrong-doing to one’s neighbour: to harm him by deed either in his own person, for it is said: Thou shalt not kill; or in a person united to him, for it is said: Thou shalt not commit adultery, or in external things, for it is said: Thou shalt not steal. We are also forbidden to wrong our neighbour by word: for it is written: Thou shalt not bear false witness against thy neighbour. And since God is judge also of hearts, we are forbidden to injure our neighbour in thought, by coveting his wife or his goods.

Now man is moved in two ways to observe this justice that is prescribed by the divine law: first, from within; secondly, from without. From within, when man is willing to observe the precepts of the divine law. This is the result of man’s love of God and his neighbour: for whoso loves another, gives him his due willingly and with pleasure, and gives even more with liberality. Hence the entire fulfilment of the law depends on love, according to the word of the Apostle (Rom. xiii. 10): Love is the fulfilling of the law. Again, our Lord says (Matth. xxii. 40) that on... two commandments,—i.e., on the love of God and our neighbour, defiendeth the whole law. But as some have not the inward disposition to do what the law commands, willingly and of their own accord, they need to be drawn from without to fulfil the justice of the law. This is done when they fulfil the law through fear of punishment, not as freemen but as slaves. Hence it is said (Isa. xxvi. 9): When thou shalt do thy judgements on the earth, namely, by punishing evil-doers, the inhabitants of the world shall learn justice.

The former, then, are a law to themselvesf for they have charity, which moves them instead of the law, and makes them act as men who are free. Wherefore it was necessary for the external law to be made not on their account, but on account of those who are not inclined to well-doing of their own accord. For this reason it is said (1 Tim. i. 9): The law is not made for the just man, but for the unjust. But this does not mean that the just are not bound to fulfd the law, as some have wrongly understood; but that they are inclined of their own accord to do justice, even without the law.