Plato, Letters 340b

ἐπειδὴ δὲ ἀφικόμην, ᾤμην τούτου πρῶτον ἔλεγχον δεῖν λαβεῖν, πότερον ὄντως εἴη Διονύσιος ἐξημμένος ὑπὸ φιλοσοφίας ὥσπερ πυρός, ἢ μάτην ὁ πολὺς οὗτος ἔλθοι λόγος Ἀθήναζε. ἔστιν δή τις τρόπος τοῦ περὶ τὰ τοιαῦτα πεῖραν λαμβάνειν οὐκ ἀγεννὴς ἀλλʼ ὄντως τυράννοις πρέπων, ἄλλως τε καὶ τοῖς τῶν παρακουσμάτων μεστοῖς, ὃ δὴ κἀγὼ Διονύσιον εὐθὺς ἐλθὼν ᾐσθόμην καὶ μάλα πεπονθότα. δεικνύναι δὴ δεῖ τοῖς τοιούτοις ὅτι ἔστι πᾶν τὸ πρᾶγμα οἷόν τε

I arrived, I deemed that I ought first of all to gain proof of this point,—whether Dionysius was really inflamed by philosophy, as it were by fire, or all this persistent account which had come to Athens was empty rumor. Now there is a method of testing such matters which is not ignoble but really suitable in the case of tyrants, and especially such as are crammed with borrowed doctrines; and this was certainly what had happened to Dionysius, as I perceived as soon as I arrived. To such persons one must point out what the subject is as a whole,