Catena Aurea on Luke 6:27–31: John Chrysostom

Now we have a natural law implanted in us, by which we distinguish between what is virtue, and what is vice. Hence it follows, And as ye would that men should do unto you, do ye also to them. He does not say, Whatever ye would not that men should do unto you, do not ye. For since there are two ways which lead to virtue, namely, abstaining from evil, and doing good, he names one, signifying by it the other also. And if indeed He had said, That ye may be men, love the beasts, the command would be a difficult one. But if they are commanded to love men, which is a natural admonition, wherein lies the difficulty, since even the wolves and lions observe it, whom a natural relation compels to love one another. It is manifest then that Christ has ordained nothing surpassing our nature, but what He had long before implanted in our conscience, so that thy own will is the law to thee. And if thou wilt have good done unto thee, thou must do good to others; if thou wilt that another should shew mercy to thee, thou must shew mercy to thy neighbour.