After retirement, he's determined not to let his mind go idle.
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From 《论语·阳货》: 子曰: '饱食终日, 无所用心, 难矣哉!' ('Confucius said: To eat one's fill all day with one's mind on nothing - how hard a case!'). The idiom usually surfaces alongside 饱食终日 ('eat one's fill all day') as a paired criticism of aimless living.