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.
AI put this 1862 translation into today's English. Only the wording changed: each rewrite was checked against the original to keep its exact meaning.
Original translation
“No longer talk at all about the kind of man that a good man ought to be, but be such.”
Translated by George Long, 1862
“Time is like a river made up of the events which happen, and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”
― Marcus AureliusMeditations (Book II, Section 8)