A passage from Marcus Aurelius
Marcus Aurelius, Meditations 9.32
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Thou canst remove out of the way many useless things among those which disturb thee, for they lie entirely in thy opinion; and thou wilt then gain for thyself ample space by comprehending the whole universe in thy mind.
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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If you are distressed by anything external, the pain is not due to the thing itself, but to your estimate of it; and this you have the power to revoke at any moment.