ζῆν οὐκ ἄξιος, ὅτῳ μηδὲ εἷς ἐστι χρηστὸς φίλος → life is not worth living if you do not have at least one friend
(see also ἐκδιδάσκω): instruct, teach, instruct in
edocere, to inform fully, 6.80.3.
