A passage from Marcus Aurelius
Marcus Aurelius, Meditations 7.2
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How can our principles become dead, unless the impressions which correspond to them are extinguished? But it is in thy power continuously to fan these thoughts into a flame.
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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Of human life the time is a point, the substance flowing, the perception dull... The only thing that escorts a man on his way is philosophy.
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The soul becomes dyed with the color of its thoughts.