A passage from Seneca
Seneca, On Tranquility of Mind VIII
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We are all bound by the law of change; we are not allowed to stay the same. The seasons pass, and we must pass with them.
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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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.
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He who acts unjustly acts impiously. For since the universal nature has made rational animals for the sake of one another, he who acts contrary to nature commits impiety against the highest divinity.