Cicero, Tusculan Disputations 1.30

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.

Do not act as if thou wert going to live ten thousand years. Death hangs over thee. While thou livest, while it is in thy power, be good.

Since it is possible that thou mayest depart from life this very moment, regulate every act and thought accordingly.