Where Slander is Lawful

Three persons only is it permissible to slander. The first is a tyrannical king who oppresses his subjects; it is lawful to speak of his misdeeds so that people may beware of him. The second is he who is shameless; deem it not a sin to speak ill of such a one, for by his own actions are his faults revealed. The third is he that gives false weight and is a cheat; say what thou knowest of his evil ways.