From St. John Chrysostom Homily on Natural law

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 to-day 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 shewing 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,” saith He, “are not necessary, nor laws of great length, nor a diversity of instruction. Let thine own will be the law. Dost thou wish to receive kindness? Be kind to another. Dost thou wish to receive mercy? Show mercy to thy neighbour. Dost thou wish to be applauded? Applaud another. Dost thou wish to be beloved? Exercise love. Dost thou wish to enjoy the first rank? First concede that place to another. Become thyself the judge, thyself the lawgiver of thine own life. And again; “Do not to another what thou hatest.” 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 saith, So say “what thou hatest.” Dost thou hate to be insulted? Do not insult another. Dost thou hate to be envied? Envy not another. Dost thou 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 hath implanted in our nature; but the practice of it and the correction He hath entrusted to our moral choice “The light of reason does not, any more than that of Revelation, force men to submit to its authority.”Perhaps what is thus said, is obscure; wherefore I will again endeavour to make it more plain. In order to know that it is a good thing to exercise temperance, we need no words, nor instruction; for we ourselves have the knowledge of it in our nature, and there is no necessity for labour or fatigue in going about and enquiring whether temperance is good and profitable; but we all acknowledge this with one consent, and no man is in doubt as to this virtue. So also we account adultery to be an evil thing, and neither is there here any need of trouble or learning, that the wickedness of this sin may be known; but we are all self-taught in such judgments; and we applaud virtue, though we do not follow it; as, on the other hand, we hate vice, though we practise it. And this hath been an exceeding good work of God; that He hath made our conscience, and our power of choice already, and before the action, claim kindred with virtue, and be at enmity with wickedness.