Demetrius, On Style 222
“Believability resides in these things, then, as in what Theophrastus says, that you should not go through everything in precise detail, but instead leave some things out for your audience to imagine and to figure out for themselves. This is because when someone imagines what you have left out, they turn out to be not only your audience but your witness too and becomes more favorably disposed for this reason.
For, thanks to you, he seems to be intelligent through your action, since you have provided him the chance to show his intelligence. Telling an audience everything as if they are stupid makes it seem like you think they are.”
(222) ἐν τούτοις τε οὖν τὸ πιθανόν, καὶ ἐν ᾧ Θεόφραστός φησιν, ὅτι οὐ πάντα ἐπ᾿ ἀκριβείας δεῖ μακρηγορεῖν, ἀλλ᾿ ἔνια καταλιπεῖν καὶ τῷ ἀκροατῇ συνιέναι καὶ λογίζεσθαι ἐξ αὑτοῦ· συνεὶς γὰρ τὸ ἐλλειφθὲν ὑπὸ σοῦ οὐκ ἀκροατὴς μόνον ἀλλὰ καὶ μάρτυς σου γίνεται, καὶ ἅμα εὐμενέστερος. συνετὸς γὰρ ἑαυτῷ δοκεῖ διὰ σὲ τὸν ἀφορμὴν παρεσχηκότα αὐτῷ τοῦ συνιέναι, τὸ δὲ πάντα ὡς ἀνοήτῳ λέγειν καταγινώσκοντι ἔοικεν τοῦ ἀκροατοῦ.