The rule given about not seeking one's own gain is established, first by the examples of Christ, next by the meaning of the word, and lastly by the very form and uses of our limbs. Wherefore the writer shows what a crime it is to deprive another of what is useful, since the law of nature as well as the divine law is broken by such wickedness. Further, by its means we also lose that gift which makes us superior to other living creatures; and lastly, through it civil laws are abused and treated with the greatest contempt.
15. If, then, any one wishes to please all, he must strive in everything to do, not what is useful for himself, but what is useful for many, as also Paul strove to do. For this is “to be conformed to the image of Christ,” namely, when one does not strive for what is another's, and does not deprive another of something so as to gain it for oneself. For Christ our Lord, though He was in the form of God, emptied Himself so as to take on Himself the form of man, which He wished to enrich with the virtue of His works. Will you, then, spoil him whom Christ has put on? Will you strip him whom Christ has clothed? For this is what you are doing when thou dost attempt to increase your own advantage at another's loss.
16. Think, O man, from whence you have received your name— even from the earth, which takes nothing from any one, but gives freely to all, and supplies varied produce for the use of all living things. Hence humanity is called a particular and innate virtue in man, for it assists its partner.
17. The very form of your body and the uses of your limbs teach you this. Can one limb claim the duties of another? Can the eye claim for itself the duties of the ear; or the mouth the duties of the eye; or the hand the service of the feet; or the feet that of the hands? Nay, the hands themselves, both left and right, have different duties to do, so that if one were to change the use of either, one would act contrary to nature. We should have to lay aside the whole man before we could change the service of the various members: as if, for instance, we were to try to take food with the left hand, or to perform the duties of the left hand with the right, so as to remove the remains of food— unless, of course, need demanded it.
18. Imagine for a moment, and give to the eye the power to withdraw the understanding from the head, the sense of hearing from the ears, the power of thought from the mind, the sense of smell from the nose, the sense of taste from the mouth, and then to assume them itself, would it not at once destroy the whole order of nature? Wherefore the Apostle says well: “If the whole body were an eye, where were the hearing? If the whole were hearing, where were the smelling?” So, then, we are all one body, though with many members, all necessary to the body. For no one member can say of another: “I have no need of you.” For those members which seem to be more feeble are much more necessary and require greater care and attention. And if one member suffers, all the members suffer with it.
19. So we see how grave a matter it is to deprive another, with whom we ought rather to suffer, of anything, or to act unfairly or injuriously towards one to whom we ought to give a share in our services. This is a true law of nature, which binds us to show all kindly feeling, so that we should all of us in turn help one another, as parts of one body, and should never think of depriving another of anything, seeing it is against the law of nature even to abstain from giving help. We are born in such a way that limb combines with limb, and one works with another, and all assist each other in mutual service. But if one fails in its duty, the rest are hindered. If, for instance, the hand tears out the eye, has it not hindered the use of its work? If it were to wound the foot, how many actions would it not prevent? But how much worse is it for the whole man to be drawn aside from his duty than for one of the members only! If the whole body is injured in one member, so also is the whole community of the human race disturbed in one man. The nature of mankind is injured, as also is the society of the holy Church, which rises into one united body, bound together in oneness of faith and love. Christ the Lord, also, Who died for all, will grieve that the price of His blood was paid in vain.
20. Why, the very law of the Lord teaches us that this rule must be observed, so that we may never deprive another of anything for the sake of our own advantage. For it says: “Remove not the bounds which your fathers have set.” It bids a neighbour's ox to be brought back if found wandering. It orders a thief to be put to death. It forbids the labourer to be deprived of his hire, and orders money to be returned without usury. It is a mark of kindly feeling to help him who has nothing, but it is a sign of a hard nature to extort more than one has given. If a man has need of your assistance because he has not enough of his own wherewith to repay a debt, is it not a wicked thing to demand under the guise of kindly feeling a larger sum from him who has not the means to pay off a less amount? Thou dost but free him from debt to another, to bring him under your own hand; and you call that human kindliness which is but a further wickedness.
21. It is in this very matter that we stand before all other living creatures, for they do not understand how to do good. Wild beasts snatch away, men share with others. Wherefore the Psalmist says: “The righteous shows mercy and gives.” There are some, however, to whom the wild beasts do good. They feed their young with what they get, and the birds satisfy their brood with food; but to men alone has it been given to feed all as though they were their own. That is so in accordance with the claims of nature. And if it is not lawful to refuse to give, how is it lawful to deprive another? And do not our very laws teach us the same? They order those things which have been taken from others with injury to their persons or property to be restored with additional recompense; so as to check the thief from stealing by the penalty, and by the fine to recall him from his ways.
22. Suppose, however, that some one did not fear the penalty, or laughed at the fine, would that make it a worthy thing to deprive another of his own? That would be a mean vice and suited only to the lowest of the low. So contrary to nature is it, that while want might seem to drive one to it, yet nature could never urge it. And yet we find secret theft among slaves, open robbery among the rich.
23. But what so contrary to nature as to injure another for our own benefit? The natural feelings of our own hearts urge us to keep on the watch for all, to undergo trouble, to do work for all. It is considered also a glorious thing for each one at risk to himself to seek the quiet of all, and to think it far more thankworthy to have saved his country from destruction than to have kept danger from himself. We must think it a far more noble thing to labour for our country than to pass a quiet life at ease in the full enjoyment of leisure.
Source: On the Duties of the Clergy (New Advent)