Plato, Letters 358c

Πλάτων Ἀριστοδώρῳ εὖ πράττειν.ἀκούω Δίωνος ἐν τοῖς μάλιστα ἑταῖρον εἶναί τέ σε νῦν καὶ γεγονέναι διὰ παντός, τὸ σοφώτατον ἦθος τῶν εἰς φιλοσοφίαν παρεχόμενον· τὸ γὰρ βέβαιον καὶ πιστὸν καὶ ὑγιές, τοῦτο ἐγώ φημι εἶναι τὴν ἀληθινὴν φιλοσοφίαν, τὰς δὲ ἄλλας τε καὶ εἰς ἄλλα τεινούσας σοφίας τε καὶ δεινότητας κομψότητας οἶμαι προσαγορεύων ὀρθῶς ὀνομάζειν. ἀλλʼ ἔρρωσό τε καὶ μένε ἐν τοῖς ἤθεσιν οἷσπερ καὶ νῦν μένεις.

Plato to Aristodorus wishes well-doing. I hear that you now are and always have been one of Dion’s most intimate companions, since of all who pursue philosophy you exhibit the most philosophic disposition; for steadfastness, trustiness, and sincerity—these I affirm to be the genuine philosophy, but as to all other forms of science and cleverness which tend in other directions, I shall, I believe, be giving them their right names if I dub them parlor-tricks. So farewell, and continue in the same disposition in which you are continuing now.