56 Hence the Lord goes on to say: “You have heard that it has been said, An eye for an eye, and a tooth for a tooth: but I say unto you, that you resist not evil; but whosoever shall smite you on your right cheek, turn to him the other also. And if any man will sue you at the law, and take away your coat [tunic, undergarment], let him have your cloak also. And whosoever shall compel you to go a mile, go with him two. Give to him that asks you, and from him that would borrow of you turn not away.” It is the lesser righteousness of the Pharisees not to go beyond measure in revenge, that no one should give back more than he has received: and this is a great step. For it is not easy to find any one who, when he has received a blow, wishes merely to return the blow; and who, on hearing one word from a man who reviles him, is content to return only one, and that just an equivalent; but he avenges it more immoderately, either under the disturbing influence of anger, or because he thinks it just, that he who first inflicted injury should suffer more severe injury than he suffered who had not inflicted injury. Such a spirit was in great measure restrained by the law, where it was written, “An eye for an eye, and a tooth for a tooth;” by which expressions a certain measure is intended, so that the vengeance should not exceed the injury. And this is the beginning of peace: but perfect peace is to have no wish at all for such vengeance.
57. Hence, between that first course which goes beyond the law, that a greater evil should be inflicted in return for a lesser, and this to which the Lord has given expression for the purpose of perfecting the disciples, that no evil at all should be inflicted in return for evil, a middle course holds a certain place, viz. that as much be paid back as has been received; by means of which enactment the transition is made from the highest discord to the highest concord, according to the distribution of times. See, therefore, at how great a distance any one who is the first to do harm to another, with the desire of injuring and hurting him, stands from him who, even when injured, does not pay back the injury. That man, however, who is not the first to do harm to any one, but who yet, when injured, inflicts a greater injury in return, either in will or in deed, has so far withdrawn himself from the highest injustice, and made so far an advance to the highest righteousness; but still he does not yet hold by what the law given by Moses commanded. And therefore he who pays back just as much as he has received already forgives something: for the party who injures does not deserve merely as much punishment as the man who was injured by him has innocently suffered. And accordingly this incomplete, by no means severe, but [rather] merciful justice, is carried to perfection by Him who came to fulfil the law, not to destroy it. Hence there are still two intervening steps which He has left to be understood, while He has chosen rather to speak of the very highest development of mercy. For there is still what one may do who does not come fully up to that magnitude of the precept which belongs to the kingdom of heaven; acting in such a way that he does not pay back as much, but less; as, for instance, one blow instead of two, or that he cuts off an ear for an eye that has been plucked out. He who, rising above this, pays back nothing at all, approaches the Lord's precept, but yet he does not reach it. For still it seems to the Lord not enough, if, for the evil which you may have received, you should inflict no evil in return, unless you be prepared to receive even more. And therefore He does not say, “But I say unto you,” that you are not to return evil for evil; although even this would be a great precept: but He says, “that you resist not evil;” so that not only are you not to pay back what may have been inflicted on you, but you are not even to resist other inflictions. For this is what He also goes on to explain: “But whosoever shall smite you on your right cheek, turn to him the other also:” for He does not say, If any man smite you, do not wish to smite him; but, Offer yourself further to him if he should go on to smite you. As regards compassion, they feel it most who minister to those whom they greatly love as if they were their children, or some very dear friends in sickness, or little children, or insane persons, at whose hands they often endure many things; and if their welfare demand it, they even show themselves ready to endure more, until the weakness either of age or of disease pass away. And so, as regards those whom the Lord, the Physician of souls, was instructing to take care of their neighbours, what else could He teach them, than that they endure quietly the infirmities of those whose welfare they wish to consult? For all wickedness arises from infirmity of mind: because nothing is more harmless than the man who is perfect in virtue.
Source: Our Lord's Sermon on the Mount (New Advent)