A passage from Marcus Aurelius
Marcus Aurelius, Meditations 8.7
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Every nature is contented with itself when it goes on its way well; and a rational nature goes on its way well, when in its thoughts it assents to nothing false or uncertain.
Related passages
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Take away thy opinion, and then there is taken away the complaint, 'I have been harmed.' Take away the complaint, and the harm is taken away.
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At dawn, when you have trouble getting out of bed, tell yourself: 'I have to go to work — as a human being.'
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Take care not to be Caesarified, or dyed with purple; for it does happen. Keep thyself then simple, good, pure, serious, free from affectation.