Plato, Letters 351d

ἐχθρῶν, θαυμαστὸν παθὼν οὐδέν. ὅσιος γὰρ ἄνθρωπος ἀνοσίων πέρι, σώφρων τε καὶ ἔμφρων, τὸ μὲν ὅλον οὐκ ἄν ποτε διαψευσθείη τῆς ψυχῆς τῶν τοιούτων πέρι, κυβερνήτου δὲ ἀγαθοῦ πάθος ἂν ἴσως οὐ θαυμαστὸν εἰ πάθοι, ὃν χειμὼν μὲν ἐσόμενος οὐκ ἂν πάνυ λάθοι, χειμώνων δὲ ἐξαίσιον καὶ ἀπροσδόκητον μέγεθος λάθοι τʼ ἂν καὶ λαθὸν κατακλύσειεν βίᾳ. ταὐτὸν δὴ καὶ Δίωνα ἔσφηλεν· κακοὶ μὲν γὰρ ὄντες αὐτὸν σφόδρα οὐκ ἔλαθον οἱ σφήλαντες, ὅσον δὲ ὕψος ἀμαθίας

For while, in dealing with the unrighteous, a righteous man who is sober and sound of mind will never be wholly deceived concerning the souls of such men; yet it would not, perhaps, be surprising if he were to share the fate of a good pilot, who, though he certainly would not fail to notice the oncoming of a storm, yet might fail to realize its extraordinary and unexpected violence, and in consequence of that failure might be forcibly overwhelmed. And Dion’s downfall was, in fact, due to the same cause; for while he most certainly did not fail to notice that those who brought him down were evil men, yet he did fail to realize to what a pitch