A passage from Seneca
Seneca, On the Happy Life 3.4
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The wise man is not lifted up by prosperity nor cast down by adversity. He bears both with equal mind, because he knows that both are external.
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.