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A saying from the Bible's story of Cain and Abel. After Cain had murdered his brother Abel, God asked him where his brother was. Cain answered, “I know not; am I my brother's keeper?”
Note: Cain's words have come to symbolize people's unwillingness to accept responsibility for the welfare of their fellows — their “brothers” in the extended sense of the term. The tradition of Judaism and Christianity is that people do have this responsibility. (See Good Samaritan, Love thy neighbor as thyself, and Love your enemies.)