A passage from Seneca
Seneca, Consolation to Helvia 12.1
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The mind is its own place, and in itself can make a heaven of hell, a hell of heaven. So too with exile: it is not the place but the mind that makes it bitter or sweet.
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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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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At dawn, when you have trouble getting out of bed, tell yourself: 'I have to go to work — as a human being.'