Then when G-d asks [Cain], ‘Where is your brother Abel?’ he arrogantly responds, ‘I do not know. Am I my brother’s keeper?’
In essence, the entire Bible is written as an affirmative response to this question.
Rabbi Joseph Telushkin, Jewish Literacy (via mermaideleh)
"Kol Yisrael arevim zeh la’zeh (All of Israel is responsible for one another)." (Shavuos 39a)