7 Perchance the tragedy of all I have told you, has greatly softened your hearts. Do not however take it amiss. For since I am about to venture upon some more subtle thoughts and require a more sensitive state of mind on your part, I have done this intentionally, in order that by the terror of the description your minds might have shaken off all listlessness, and withdrawn themselves from all worldly cares, and might with the more readiness convey the force of the things about to be spoken into the depths of your soul.
Sufficiently indeed, then, our discourse of late evinced to you, that a natural law of good and evil is seated within us. But that our proof of it may be more abundantly evident, we will again today apply ourselves strenuously to the same subject of discourse. For that God from the beginning, when He formed man, made him capable of discriminating both these, all men make evident. Hence when we sin, we are all ashamed at the presence of our inferiors; and oftentimes a master, on his way to the house of a harlot, if he then perceives any one of his more respectable servants, turns back, reddening with shame, from this untoward path. Again, when others reproach us, fixing on us the names of particular vices, we call it an insult; and if we are aggrieved, we drag those who have done the wrong to the public tribunal. Thus we can understand what vice is and what virtue is. Wherefore Christ, for the purpose of declaring this, and showing that He was not introducing a strange law, or one which surpassed our nature, but that which He had of old deposited beforehand in our conscience, after pronouncing those numerous Beatitudes, thus speaks; “All things whatsoever ye would that men should do to you, do ye even so to them.” “Many words,” says He, are not necessary, nor laws of great length, nor a diversity of instruction. Let your own will be the law. Do you wish to receive kindness? Be kind to another. Do you wish to receive mercy? Show mercy to your neighbour. Do you wish to be applauded? Applaud another. Do you wish to be beloved? Exercise love. Do you wish to enjoy the first rank? First concede that place to another. Become yourself the judge, yourself the lawgiver of your own life. And again; “Do not to another what you hate.” By the latter precept, he would induce to a departure from iniquity; by the former, to the exercise of virtue. “Do not thou to another,” he says, “what you hate.” Do you hate to be insulted? Do not insult another. Do you hate to be envied? Envy not another. Do you hate to be deceived? Do not deceive another. And, in a word, in all things, if we hold fast these two precepts, we shall not need any other instruction. For the knowledge of virtue He has implanted in our nature; but the practice of it and the correction He has entrusted to our moral choice.
Source: Homilies on the Statues (New Advent)