Time

Remember how long thou hast been putting off these things, and how often thou hast received an opportunity from the gods, and yet dost not use it. Thou must now at last perceive of what universe thou art a part, and of what administrator of the universe thy existence is an efflux, and that a limit of time is fixed for thee, which if thou dost not use for clearing away the clouds from thy mind, it will go and thou wilt go, and it will never return.

~ Marcus Aurelius, Meditations (http://classics.mit.edu/Antoninus/meditations.2.two.html)