A passage from Marcus Aurelius
Marcus Aurelius, Meditations 3.16
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Let not a man do anything else but what tends to the harmony of the whole, and what is useful to it. For every part of nature, if it is useful, it is also good; and it is useful for the whole.
Related passages
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Begin the morning by saying to thyself, I shall meet with the busy-body, the ungrateful, arrogant, deceitful, envious, unsocial.
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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.