Meditations 2.8

Through not observing what is in the mind of another a man has seldom been seen to be unhappy; but those who do not observe the movements of their own minds must of necessity be unhappy.

Marcus Aurelius, Meditations 2.8, translated by George Long

Not easily is a man found to be unhappy by reason of his not regarding what is going on in another man's soul; but those who do not attend closely to the motions of their own souls must inevitably be unhappy.

Marcus Aurelius, Meditations 2.8, translated by C. R. Haines