A passage from Epictetus
Epictetus, Discourses III.1
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Of all the things that are, some are in our power, and some are not. In our power are opinion, desire, aversion, and in a word, whatever are our own acts. Not in our power are body, property, reputation, office, and in a word, whatever are not our own acts.
Related passages
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Men seek retreats for themselves, houses in the country, sea-shores, and mountains... But this is altogether a mark of the most common sort of men, for it is in thy power whenever thou shalt choose to retire into thyself.
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Take away thy opinion, and then there is taken away the complaint, 'I have been harmed.' Take away the complaint, and the harm is taken away.
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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.'