Remember also that every man lives only this present moment, which is a fleeting instant: the rest of time is either spent or quite unknown.[Kαὶ ἔτι συμμνημόνευε, ὅτι μόνον ζῇ ἕκαστος τὸ παρὸν τοῦτο, τὸ ἀκαριαῖον: τὰ δὲ ἄλλα ἢ βεβίωται ἢ ἐν ἀδήλῳ.]

Source:Meditations (ed. Penguin, 1964), (tr. Staniforth), Book 3, #10, Page 59
Find more on