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What is the meaning of "Nefesh Tachas Nafesh"?
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Ramban: It means (not literally that one kills the striker's animal, but) that he must pay to replace the animal that he killed.
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Ramban: Likewise, in Pasuk 20 "Shever Tachas Shever, Ayin Tachas Ayin ... " - it means that the striker is obligated to pay for the damage.