3. _Not to return evil for good._--Here we rise, in some respect, a degree in the moral scale. The two inferior degrees, namely, ingratitude and cruelty, have always and everywhere been considered as crimes. Nowhere has it ever been considered allowable to do harm to those who have done us good. But in nearly all societies, at a certain degree of civilization, has it been considered allowable, and even praiseworthy, to return evil for evil. "To do good to our friends, and harm to our enemies," is one of the maxims the poets and sages of Greece oftenest repeat. Among the Indians of America, glory consists in ornamenting one's dwelling with the greatest possible number of scalps taken from conquered enemies. We know about the Corsican _vendetta_. In one word, the passion of revenge (which consists precisely in returning evil for evil) is one of the most natural and the most profound in the human heart, and it demands a very advanced moral education to comprehend that revenge is contrary to the laws of morality. Now, as the beauty of virtue is in proportion to the difficulty of the passions to be overcome, it is evident that the virtues contrary to revenge, namely: _gentleness_, _clemency_, _pardon of injuries_, are amongst the most beautiful and most sublime. Already among the ancients had morality reached this maxim, that one should not do any harm, namely, even to those who had done us some, as may be seen from the dialogue of Plato, entitled the _Crito_. "_Socrates_: One should then commit no injustice whatsoever?" "_Crito_: No, certainly not." "_Socrates_: Then should one not be unjust even towards those who are unjust towards us."