A passage from Seneca
Seneca, On Tranquility of Mind XII
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The best way to be tranquil is to do nothing that is not useful, and to do everything that is useful, with a calm and cheerful mind.
Related passages
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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.
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It is a man's special privilege to love even those who wrong him.