74 Moreover, a certain strength and vigour in walking along the path of wisdom ties in good morals, which are made to extend as far as to purification and singleness of heart—a subject on which He has now been speaking long, and thus concludes: “Therefore all good things whatsoever ye would that men should do to you, do ye even so to them: for this is the law and the prophets.” In the Greek copies we find the passage runs thus: “Therefore all things whatsoever ye would that men should do to you, do ye even so to them.” But I think the word “good” has been added by the Latins to make the sentence clear. For the thought occurred, that if any one should wish something wicked to be done to him, and should refer this clause to that—as, for instance, if one should wish to be challenged to drink immoderately, and to get drunk over his cups, and should first do this to the party by whom he wishes it to be done to himself—it would be ridiculous to imagine that he had fulfilled this clause. Inasmuch, therefore, as they were influenced by this consideration, as I suppose, one word was added to make the matter clear; so that in the statement, “Therefore all things whatsoever ye would that men should do to you,” there was inserted the word “good.” But if this is wanting in the Greek copies, they also ought to be corrected: but who would venture to do this? It is to be understood, therefore, that the clause is complete and altogether perfect, even if this word be not added. For the expression used, “whatsoever ye would,” ought to be understood as used not in a customary and random, but in a strict sense. For there is no will except in the good: for in the case of bad and wicked deeds, desire is strictly spoken of, not will. Not that the Scriptures always speak in a strict sense; but where it is necessary, they so keep a word to its perfectly strict meaning, that they do not allow anything else to be understood.
75. Moreover, this precept seems to refer to the love of our neighbour, and not to the love of God also, seeing that in another passage He says that there are two precepts on which “hang all the law and the prophets.” For if He had said, All things whatsoever ye would should be done to you, do ye even so; in this one sentence He would have embraced both those precepts: for it would soon be said that every one wishes that he himself should be loved both by God and by men; and so, when this precept was given to him, that what he wished done to himself he should himself do, that certainly would be equivalent to the precept that he should love God and men. But when it is said more expressly of men, “Therefore all things whatsoever ye would that men should do to you, do ye even so to them,” nothing else seems to be meant than, “You shall love your neighbour as yourself.” But we must carefully attend to what He has added here: “for this is the law and the prophets.” Now, in the case of these two precepts, He not merely says, The law and the prophets hang; but He has also added, “all the law and the prophets,” which is the same as the whole of prophecy: and in not making the same addition here, He has kept a place for the other precept, which refers to the love of God. Here, then, inasmuch as He is following out the precepts with respect to a single heart, and it is to be dreaded lest any one should have a double heart toward those from whom the heart can be hid, i.e. toward men, a precept with respect to that very thing was to be given. For there is almost nobody that would wish that any one of double heart should have dealings with himself. But no one can bestow anything upon a fellowman with a single heart, unless he so bestow it that he expects no temporal advantage from him, and does it with the intention which we have sufficiently discussed above, when we were speaking of the single eye.
76. The eye, therefore, being cleansed and rendered single, will be adapted and suited to behold and contemplate its own inner light. For the eye in question is the eye of the heart. Now, such an eye is possessed by him who, in order that his works may be truly good, does not make it the aim of his good works that he should please men; but even if it should turn out that he pleases them, he makes this tend rather to their salvation and to the glory of God, not to his own empty boasting; nor does he do anything that is good tending to his neighbour's salvation for the purpose of gaining by it those things that are necessary for getting through this present life; nor does he rashly condemn a man's intention and wish in that action in which it is not apparent with what intention and wish it has been done; and whatever kindnesses he shows to a man, he shows them with the same intention with which he wishes them shown to himself, viz. as not expecting any temporal advantage from him: thus will the heart be single and pure in which God is sought. “Blessed,” therefore, “are the pure in heart: for they shall see God.”
Source: Our Lord's Sermon on the Mount (New Advent)