Plato, Laws 665a

τάξει ῥυθμὸς ὄνομα εἴη, τῇ δὲ αὖ τῆς φωνῆς, τοῦ τε ὀξέος ἅμα καὶ βαρέος συγκεραννυμένων, ἁρμονία ὄνομα προσαγορεύοιτο, χορεία δὲ τὸ συναμφότερον κληθείη. θεοὺς δὲ ἔφαμεν ἐλεοῦντας ἡμᾶς συγχορευτάς τε καὶ χορηγοὺς ἡμῖν δεδωκέναι τόν τε Ἀπόλλωνα καὶ μούσας, καὶ δὴ καὶ τρίτον ἔφαμεν, εἰ μεμνήμεθα, Διόνυσον. πῶς δʼ οὐ μεμνήμεθα; ὁ μὲν τοίνυν τοῦ Ἀπόλλωνος καὶ τῶν Μουσῶν

of motion is called rhythm, while the order of voice (in which acute and grave are blended together) is termed harmony, and to the combination of these two the name choristry is given. We stated also that the gods, in pity for us, have granted to us as fellow-choristers and choir-leaders Apollo and the Muses,—besides whom we mentioned, if we recollect, a third, Dionysus. Certainly we recollect. The choir of Apollo and that of the Muses have been described, and the third and remaining