Plato, Epinomis 991b

δώδεκα συνέβη τό τε ἡμιόλιον καὶ ἐπίτριτον—τούτων αὐτῶν ἐν τῷ μέσῳ ἐπʼ ἀμφότερα στρεφομένη τοῖς ἀνθρώποις σύμφωνον χρείαν καὶ σύμμετρον ἀπενείματο παιδιᾶς ῥυθμοῦ τε καὶ ἁρμονίας χάριν, εὐδαίμονι χορείᾳ Μουσῶν δεδομένη. ταῦτα μὲν οὖν δὴ ταύτῃ γιγνέσθω τε καὶ ἐχέτω σύμπαντα· τὸ δʼ ἐπὶ τούτοις τέλος, εἰς θείαν γένεσιν ἅμα καὶ τὴν τῶν ὁρατῶν καλλίστην τε καὶ θειοτάτην φύσιν ἰτέον, ὅσην ἀνθρώποις θεὸς ἔδωκεν κατιδεῖν, ἣν οὔποτε ἄνευ τῶν νῦν

twelve comes the whole-and-a-half (9 = 6 + 3) and whole-and-a-third (8 = 6 + 2): turning between these very two, to one side or the other, this power (9) assigned to men an accordant and proportioned use for the purpose of rhythm and harmony in their pastimes, and has been assigned to the blessed dance of the Muses. In this way then let all these things come to pass, and so let them be. But as to their crowning point, we must go to divine generation and therewith the fairest and divinest nature of visible things, so far as God granted the vision of it to men; a vision that none of us may ever boast of having received at his leisure