A passage from Seneca
Seneca, Letters to Lucilius Letter 1
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Nothing, Lucilius, is ours, except time... Hold every hour in your grasp. Lay hold of today's task, and you will not need to depend so much upon tomorrow's.
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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At dawn, when you have trouble getting out of bed, tell yourself: 'I have to go to work — as a human being.'
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Waste no more time arguing what a good man should be. Be one.