A passage from Epictetus
Epictetus, Discourses IV.4
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The good man is invincible, for he does not enter into contests where he is not superior. If he is not strong enough to conquer an opponent, he will not enter into the contest.
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.