A man must know the truth about all the particular things of which he speaks or writes, and must be able to define everything separately; then when he has defined them, he must know how to divide them by classes until further division is impossible; and in the same way he must understand the nature of the soul, must find out the class of speech adapted to each nature, and must arrange and adorn his discourse accordingly, offering to the complex soul elaborate and harmonious discourses, and simple talks to the simple soul. Until he has attained to all this, he will not be able to speak by the method of art, so far as speech can be controlled by method, either for purposes of instruction or of persuasion. This has been taught by our whole preceding discussion.

Πρὶν ἄν τις τό τε ἀληθὲς ἑκάστων εἰδῇ πέρι ὧν λέγει ἢ γράφει, κατ’ αὐτό τε πᾶν ὁρίζεσθαι δυνατὸς γένηται, ὁρισάμενός τε πάλιν κατ’ εἴδη μέχρι τοῦ ἀτμήτου τέμνειν ἐπιστηθῇ, περί τε ψυχῆς φύσεως διιδὼν κατὰ ταὐτά, τὸ προσαρμόττον ἑκάστῃ φύσει εἶδος ἀνευρίσκων, οὕτω τιθῇ καὶ διακοσμῇ τὸν λόγον, ποικίλῃ μὲν ποικίλους ψυχῇ καὶ παναρμονίους διδοὺς λόγους, ἁπλοῦς δὲ ἁπλῇ, οὐ πρότερον δυνατὸν τέχνῃ ἔσεσθαι καθ’ ὅσον πέφυκε μεταχειρισθῆναι τὸ λόγων γένος, οὔτε τι πρὸς τὸ διδάξαι οὔτε τι πρὸς τὸ πεῖσαι, ὡς ὁ ἔμπροσθεν πᾶς μεμήνυκεν ἡμῖν λόγος.

Plato, Phaedrus 277b-c. (tr. Fowler)