A passage from Marcus Aurelius
Marcus Aurelius, Meditations 7.27
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Think not so much of what thou hast not as of what thou hast: but of the things which thou hast select the best, and then reflect how eagerly they would have been sought if thou hadst them not.
Related passages
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Your present opinion founded on understanding, your present conduct directed to social good, your present disposition contented with everything that happens — that is enough.
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Everything you're trying to reach — by taking the long way round — you could have right now, this moment. If you'd only stop thwarting your own attempts.
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Every nature is contented with itself when it goes on its way well; and a rational nature goes on its way well, when in its thoughts it assents to nothing false or uncertain.