Meditations 2.8.1
Marcus Aurelius to himself
Παρὰ μὲν τὸ μὴ ἐφιστάνειν, τί ἐν τῇ ἄλλου ψυχῇ γίνεται, οὐ ῥᾳδίως τις ὤφθη κακοδαιμονῶν· τοὺς δὲ τοῖς τῆς ἰδίας ψυχῆς κινήμασι μὴ παρακολουθοῦντας ἀνάγκη κακοδαιμονεῖν.
People have rarely been seen miserable because they failed to notice what was happening in another person’s soul. But those who fail to follow the movements of their own soul must be miserable.