Plato, Letters 344b

τὸ δυνατὸν οὐδὲ κακίας. ἅμα γὰρ αὐτὰ ἀνάγκη μανθάνειν καὶ τὸ ψεῦδος ἅμα καὶ ἀληθὲς τῆς ὅλης οὐσίας, μετὰ τριβῆς πάσης καὶ χρόνου πολλοῦ, ὅπερ ἐν ἀρχαῖς εἶπον· μόγις δὲ τριβόμενα πρὸς ἄλληλα αὐτῶν ἕκαστα, ὀνόματα καὶ λόγοι ὄψεις τε καὶ αἰσθήσεις, ἐν εὐμενέσιν ἐλέγχοις ἐλεγχόμενα καὶ ἄνευ φθόνων ἐρωτήσεσιν καὶ ἀποκρίσεσιν χρωμένων, ἐξέλαμψε φρόνησις περὶ ἕκαστον καὶ νοῦς, συντείνων ὅτι

the truth of virtue nor yet of vice. For in learning these objects it is necessary to learn at the same time both what is false and what is true of the whole of Existence, and that through the most diligent and prolonged investigation, as I said at the commencement ; and it is by means of the examination of each of these objects, comparing one with another—names and definitions, visions and sense-perceptions,—proving them by kindly proofs and employing questionings and answerings that are void of envy—it is by such means, and hardly so, that there bursts out the light of intelligence and reason regarding each object in the mind of him who uses every effort of which mankind is capable.