The Talmud quotes a sage who tells us that knowledge and understanding (de'a) is "great," meaning, of prime importance and for that reason it is the subject of the first of the weekday petitions in the Amida prayer (B. Brachot 33a). A further proof of the significance of knowledge and understanding is that it appears in a biblical verse between two divine names: For the God of thoughts (de'ot) is the Almighty (I Samuel 2:3). Following this method of identifying premier values, another sage suggests a further item of significant weight: The Temple is "great" for it appears between two divine names in the Song at the Sea: The foundation of the dwelling place You have made, O God; the Temple, O God, that Your hands have established (Exodus 15:17). Moreover the two values - knowledge and the Temple - are understood to be intrinsically connected: "Whoever has knowledge and understanding (de'a) it is as if the Temple was built in his days," for both values appear in the Bible between two divine names. One sage was driven to question the reliability of identifying major values by their appearance in the biblical text between two divine names: "According to this you would have to say that revenge is also great for it too was placed between two divine names - Almighty one of vengeance, O God of vengeance, appear" (Psalms 94:1). The Talmud concedes that vengeance is indeed great in appropriate circumstances, but not all vengeance is worthy. The biblical verse, as we have just seen, mentions vengeance twice: At times revenge is called for and that type of vengeance is surrounded by two divine names; other vengeance is evil and should be obliterated. Despite acknowledging the possibility of worthy vengeance, the rule appears to eschew revenge; the Torah instructs: You shall not avenge, nor bear any grudge against the children of your people, and you should love your neighbor as yourself; I am God (Leviticus 19:18). No quarter is given in this biblical verse to the option of vengeance. What type of vengeance could be considered worthy? The Talmud finds an example from the divine realm: God's vengeance against the nations of the world who rejected Torah is a worthy revenge. Though revenge is generally shunned, we are not enjoined to merely stand by silently if we are disparaged. In his 13th-century enumeration of the 613 commandments, the unknown author of Sefer Hahinuch rejects the notion that the command against revenge proscribes any type of response: "If someone comes and wickedly pains his fellow with evil words, the intention is not that the vilified person should not respond, for it is impossible for a person to be like a rock that never moves. Moreover, if he is silent it is as if he is admitting to the defamatory attack. In truth, the Torah did not command that a person should be like a rock - silent to those who abuse him as he is silent to those who bless him." The Hinuch continues comparing a verbal response to a defamatory attack to legitimate self-defense in the face of a physical assault. The prohibition against revenge does not include a prohibition against self-defense, thus a person is permitted to respond to attacks, though such a response does not include revenge. We have seen that barring divine vengeance and legitimate self-defense, classic revenge is shunned. What is so bad about vengeance? One biblical commentator, the Netziv (19th century, Volozhin, today in Belarus), offered a social reason for avoiding revenge. He explained that the act of revenge often begins a vicious cycle, where each vengeful act is reciprocated by another vengeful act. This approach echoed an early passage in Maimonides's halachic magnum opus where he concluded the laws against revenge and bearing a grudge with the lofty words: "And this - that is, not taking revenge or bearing a grudge - is the correct ethic that will enable the world to be settled and people to have dealings with each other." So important was the injunction against revenge and bearing a grudge in the eyes of Maimonides that he included it toward the beginning of his work where he deals with matters of character refinement, rather than be discussed as a subset of the laws of damages. The Netziv advocated another path, as embodied in a different biblical verse: If your enemy be hungry, give him bread to eat; and if he be thirsty, give him water to drink. For you will heap coals of fire on his head - and thus prevent him from reacting badly toward you in the future - and God will reward you (Proverbs 25:21-22). In succinct but lucid terms the Talmud illustrates the folly of revenge in different terms (J. Nedarim 41c): You are cutting meat, and by mistake your right hand cuts your left hand. Would your left hand pick up the knife and chop off your right hand?! As a people we are one body. Taking revenge against a fellow is as silly as one hand taking revenge against another; in the end the body has just incurred more injuries. As we internalize that taking revenge is only harming ourselves, we will quickly shy from this path. The prohibition against revenge reflects the joint and reciprocal relationship we have with each other. Revenge is a dangerous tool. While it may be permitted in delineated circumstances, it should be avoided. To be sure, we are not commanded to take a path of nonresistance in the face of assaults - physical or verbal - from an aggressor. Yet our response should be checked for a spiteful revenge reflects a lack of brotherhood and in truth the avenger really harms himself. Perhaps this is what the Talmud means when it says that vengeance is "great" - not great in the celebratory sense, but powerful and dangerous tool. An act that is directed at another, an act that appears not to affect the perpetrator, an act that is aimed at harming another, in truth harms the avenger. The writer is on the faculty of Pardes Institute of Jewish Studies and is a rabbi in Tzur Hadassah.