When Heaven is about to give a great task to such a person, it must first make his mind and will suffer.
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From Mencius 《孟子·告子下》: '故天将降大任于是人也，必先苦其心志，劳其筋骨，饿其体肤，空乏其身，行拂乱其所为.' (Therefore when Heaven is about to give a great mission to such a person, it first torments his mind and will, tires his muscles and bones, starves his body, impoverishes him, and confounds his undertakings.) The list of trials that makes a hero.
usage
苦 is causative here: 'cause to suffer.' 心志 = mind and will. Almost always cited as part of the longer Mencius sequence.
