Forget About All Non-Historical Speech

Diodorus Siculus, Library of History 1.2:

The study of history also enhances the power of speech, and it would not be easy to discover something finer than that. For it is in this respect that the Greeks differ from the barbarians, and it is thus that the educated gain their ascendancy over the uneducated, and further, it is through speech alone that one person is able to be superior to many. On the whole, every idea which is proposed appears only as great as the power of the speaker makes it, and we call good men “worthy of speech,” as though they had thus earned the first title of virtue.

Dividing speech into more parts, it happens that poetic speech delights more than it helps, while law codes punish but do not teach, and further, many parts of speech do nothing to enhance our happiness, others have some admixture of harm with the benefit they confer, and not a few convert truth to falsehood. Only history, when words agree with the deeds they describe, embraces all useful things in writing. For one may see it turned toward justice, chastising the idle, praising the good, and on the whole making the greatest fund of experience for those who happen upon it.

συμβάλλεται δ᾽ αὕτη καὶ πρὸς λόγου δύναμιν, οὗ κάλλιον ἕτερον οὐκ ἄν τις ῥᾳδίως εὕροι. τούτῳ γὰρ οἱ μὲν Ἕλληνες τῶν βαρβάρων, οἱ δὲ πεπαιδευμένοι τῶν ἀπαιδεύτων προέχουσι, πρὸς δὲ τούτοις διὰ μόνου τούτου δυνατόν ἐστιν ἕνα τῶν πολλῶν περιγενέσθαι: καθόλου δὲ φαίνεται πᾶν τὸ προτεθὲν τοιοῦτον ὁποῖον ἄν ἡ τοῦ λέγοντος δύναμις παραστήσῃ, καὶ τοὺς ἀγαθοὺς ἄνδρας ἀξίους λόγου προσαγορεύομεν, ὡς τοῦτο τὸ πρωτεῖον τῆς ἀρετῆς περιπεποιημένους. εἰς πλείω δὲ μέρη τούτου διῃρημένου, συμβαίνει τὴν μὲν ποιητικὴν τέρπειν μᾶλλον ἤπερ ὠφελεῖν, τὴν δὲ νομοθεσίαν κολάζειν, οὐ διδάσκειν, παραπλησίως δὲ καὶ τἄλλα μέρη τὰ μὲν μηδὲν συμβάλλεσθαι πρὸς εὐδαιμονίαν, τὰ δὲ μεμιγμένην ἔχειν τῷ συμφέροντι τὴν βλάβην, ἔνια δὲ κατεψεῦσθαι τῆς ἀληθείας, μόνην δὲ τὴν ἱστορίαν, συμφωνούντων ἐν αὐτῇ τῶν λόγων τοῖς ἔργοις, ἅπαντα τἄλλα χρήσιμα τῇ γραφῇ περιειληφέναι. ὁρᾶσθαι γὰρ αὐτὴν προτρεπομένην ἐπὶ δικαιοσύνην, κατηγοροῦσαν τῶν φαύλων, ἐγκωμιάζουσαν τοὺς ἀγαθούς, τὸ σύνολον ἐμπειρίαν μεγίστην περιποιοῦσαν τοῖς ἐντυγχάνουσι.