How the Law of God relates a man to his Neighbour

OF all things that man makes use of, the chief are other men. Man is naturally a social animal, needing many things that the individual cannot procure by himself. The divine law therefore must needs instruct man to live according to the order of reason in his relations with other men.

2. The end of the divine law is to bring man to cleave to God. Now man is aided thereto by his fellow-man, as well in point of knowledge as in point of affection: for men help one another in the knowledge of the truth, and one incites another to good and restrains him from evil. Hence it is said: Iron is sharpened by iron, and man sharpens the face of his friend (Prov. xxvii, 17): Better two together than one, etc. (Eccles iv, 9-12).

There is then orderly concord amongst men, when to each there is rendered his own, which is the act of justice; and therefore it is said: The work of justice is peace (Isa. xxxii, 17). To the observance of this justice man is inclined both by an interior and an exterior principle. By an interior principle, in so far as a man has a will to observe the precepts of the divine law, which is done by his bearing love to God and to his neighbour: for whoever loves another renders him his due spontaneously and with pleasure, and even acts more by liberality: hence the whole fulfilment of the law hinges upon love (Rom. xiii, 10: Matt. xxii, 40). But because some are not so inwardly disposed as to do of their own accord what the law commands, they have to be dragged by an exterior force to the fulfilment of the justice of the law; and so they fulfil the law under fear of penalties, not as freemen but as slaves. Hence it is said: When thou shalt do thy judgements upon the earth by punishing the wicked, the inhabitants of earth shall learn justice (Isa. xxvi, 9). Others are so disposed as to do of their own accord what the law bids them. They are a law to themselves, having charity, which bends their wills in place of a law to generous conduct. There was no need of an exterior law being enacted for them: hence it is said: The law was not made for the just, but for the unjust: which is not to be taken to mean that the just are not bound to fulfil the law, as some have misunderstood the text, but that the just are inclined of themselves to do justice even without a law.

3.127 : That of no Food is the Use Sinful in itself
3.129 : That the things commanded by the Divine Law are Right, not only because the Law enacts them, but also according to Nature