The Talmud (Shevuot 39a), in discussing the domino effect of sin, concludes with the Aramaic phrase, Kol yisrael arevim zeh bazeh, meaning all of Israel are responsible for each other. 
This phrase is the basis of the notion of communal responsibility in Jewish law. If one Jew sees another Jew at the verge of sinning, he has an obligation to step in and help. Even more so, it implies an obligation on all Jews to ensure that other Jews have their basic needs for food, clothing, and shelter taken care of. Simply by virtue of being a Jew one is responsible for the well-being of other Jews, and vice versa.
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