False words are not only evil in themselves, but they infect the soul with evil.

literally: 'For know well', he said, 'o dearest Kriton, that to not speak well is not only sinful by itself, but lets evil intrude into the soul.'(εὖ γὰρ ἴσθι, ἦ δ᾽ ὅς, ὦ ἄριστε Κρίτων, τὸ μὴ καλῶς λέγειν οὐ μόνον εἰς αὐτὸ τοῦτο πλημμελές, ἀλλὰ καὶ κακόν τι ἐμποιεῖ ταῖς ψυχαῖς.)