As it has been shown that he who injures another for the sake of his own advantage will undergo terrible punishment at the hand of his own conscience, it is referred that nothing is useful to one which is not in the same way useful to all. Thus there is no place among Christians for the question propounded by the philosophers about two shipwrecked persons, for they must show love and humility to all.
24. Hence we infer that a man who guides himself according to the ruling of nature, so as to be obedient to her, can never injure another. If he injures another, he violates nature, nor will he think that what he has gained is so much an advantage as a disadvantage. And what punishment is worse than the wounds of the conscience within? What judgment harder than that of our hearts, whereby each one stands convicted and accuses himself of the injury that he has wrongfully done against his brother? This the Scriptures speak of very plainly, saying: “Out of the mouth of fools there is a rod for wrong-doing.” Folly, then, is condemned because it causes wrong-doing. Ought we not rather to avoid this, than death, or loss, or want, or exile, or sickness? Who would not think some blemish of body or loss of inheritance far less than some blemish of soul or loss of reputation?
25. It is clear, then, that all must consider and hold that the advantage of the individual is the same as that of all, and that nothing must be considered advantageous except what is for the general good. For how can one be benefited alone? That which is useless to all is harmful. I certainly cannot think that he who is useless to all can be of use to himself. For if there is one law of nature for all, there is also one state of usefulness for all. And we are bound by the law of nature to act for the good of all. It is not, therefore, right for him who wishes the interests of another to be considered according to nature, to injure him against the law of nature.
26. For if those who run in a race are, as one hears, instructed and warned each one to win the race by swiftness of foot and not by any foul play, and to hasten on to victory by running as hard as they can, but not to dare to trip up another or push him aside with their hand, how much more in the course of this life ought the victory to be won by us, without falseness to another and cheating?
27. Some ask whether a wise man ought in case of a shipwreck to take away a plank from an ignorant sailor? Although it seems better for the common good that a wise man rather than a fool should escape from shipwreck, yet I do not think that a Christian, a just and a wise man, ought to save his own life by the death of another; just as when he meets with an armed robber he cannot return his blows, lest in defending his life he should stain his love toward his neighbour. The verdict on this is plain and clear in the books of the Gospel. “Put up your sword, for every one that takes the sword shall perish with the sword.” What robber is more hateful than the persecutor who came to kill Christ? But Christ would not be defended from the wounds of the persecutor, for He willed to heal all by His wounds.
28. Why do you consider yourself greater than another, when a Christian man ought to put others before himself, to claim nothing for himself, usurp no honours, claim no reward for his merits? Why, next, are you not wont to bear your own troubles rather than to destroy another's advantage? For what is so contrary to nature as not to be content with what one has or to seek what is another's, and to try to get it in shameful ways. For if a virtuous life is in accordance with nature— for God made all things very good— then shameful living must be opposed to it. A virtuous and a shameful life cannot go together, since they are absolutely severed by the law of nature.
Source: On the Duties of the Clergy (New Advent)