A passage from Marcus Aurelius
Marcus Aurelius, Meditations 4.27
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To the better part of the soul it is indifferent whether it is moved to a feverish or a healthy state, or whether it is in a state of heat or cold; for the soul is equally indifferent to all these.
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.'