"In revenge there is something which satisfies one's sense of justice. The avenger reasons:--"My good father did not deserve death. He who killed him did great evil. My father, if he were alive, would not tolerate a deed like this: Heaven itself hates wrong-doing. It is the will of my father; it is the will of Heaven that the evil-doer cease from his work. He must perish by my hand; because he shed my father's blood, I, who am his flesh and blood, must shed the murderer's. The same Heaven shall not shelter him and me." The ratiocination is simple and childish (though we know Hamlet did not reason much more deeply), nevertheless it shows an innate sense of exact balance and equal justice 'An...

"In revenge there is something which satisfies one's sense of justice. The avenger reasons:--"My good father did not deserve death. He who killed him did great evil. My father, if he were alive, would not tolerate a deed like this: Heaven itself hates wrong-doing. It is the will of my father; it is the will of Heaven that the evil-doer cease from his work. He must perish by my hand; because he shed my father's blood, I, who am his flesh and blood, must shed the murderer's. The same Heaven shall not shelter him and me." The ratiocination is simple and childish (though we know Hamlet did not reason much more deeply), nevertheless it shows an innate sense of exact balance and equal justice 'An eye for an eye, a tooth for a tooth.' Our sense of revenge is as exact as our mathematical faculty, and until both terms of the equation are satisfied we cannot get over the sense of something left undone."

Inazo Nitobe (Bushido: The Soul of Japan)