A passage from Epictetus
Epictetus, Discourses I.7
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If you are a philosopher, you will do what you do not like to do when reason requires it; if you are not a philosopher, you will do what you like to do, even if it is the worst of actions.
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.