A passage from Epictetus
Epictetus, Discourses II.20
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When you do anything, first ask yourself, Is this in accordance with my nature? Is it in accordance with the nature of the universe? Then do it, and never regret it.
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.