Demetrius, On Style 223-224
“Then we will talk about the style of a letter which also needs to be simple. Artemôn who edited the letters of Aristotle, says that we ought to write letters the same way as dialogues since a letter is similar to one side of a dialogue.
Perhaps he speaks the truth in this, but not all of it. For a letter ought to be a little more formalized than a dialogue. For a dialogue imitates speaking extemporaneously while a letter is written and sent as a gift in some way.”
(223) Ἐπεὶ δὲ καὶ ὁ ἐπιστολικὸς χαρακτὴρ δεῖται ἰσχνότητος, καὶ περὶ αὐτοῦ λέξομεν. Ἀρτέμων μὲν οὖν ὁ τὰς Ἀριστοτέλους ἀναγράψας ἐπιστολάς φησιν, ὅτι δεῖ ἐν τῷ αὐτῷ τρόπῳ διάλογόν τε γράφειν καὶ ἐπιστολάς· εἶναι γὰρ τὴν ἐπιστολὴν οἷον τὸ ἕτερον μέρος τοῦ διαλόγου.
(224) καὶ λέγει μέν τι ἴσως, οὐ μὴν ἅπαν· δεῖ γὰρ ὑποκατεσκευάσθαι πως μᾶλλον τοῦ διαλόγου τὴν ἐπιστολήν· ὁ μὲν γὰρ μιμεῖται αὐτοσχεδιάζοντα, ἡ δὲ γράφεται καὶ δῶρον πέμπεται τρόπον τινά.