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.

-Meditation 2:8

Others are unknowable, you know this. It should not sadden you. But not knowing yourself? That’s a problem.

Like many of the meditations, its easy to read this as a call to meditation. Meditation, as we conceive it, would likely have been foreign to Aurelius, but a certain type of mindfulness? Of self-knowledge? I think that is what he is getting at here.