On Christian Doctrine, in Four Books

Chapter 17

27. For we must not listen to the falsities of heathen superstition, which represent the nine Muses as daughters of Jupiter and Mercury. Varro refutes these, and I doubt whether any one can be found among them more curious or more learned in such matters. He says that a certain state (I don't recollect the name) ordered from each of three artists a set of statues of the Muses, to be placed as an offering in the temple of Apollo, intending that whichever of the artists produced the most beautiful statues, they should select and purchase from him. It so happened that these artists executed their works with equal beauty, that all nine pleased the state, and that all were bought to be dedicated in the temple of Apollo; and he says that afterwards Hesiod the poet gave names to them all. It was not Jupiter, therefore, that begat the nine Muses, but three artists created three each. And the state had originally given the order for three, not because it had seen them in visions, nor because they had presented themselves in that number to the eyes of any of the citizens, but because it was obvious to remark that all sound, which is the material of song, is by nature of three kinds. For it is either produced by the voice, as in the case of those who sing with the mouth without an instrument; or by blowing, as in the case of trumpets and flutes; or by striking, as in the case of harps and drums, and all other instruments that give their sound when struck.