Harmony

At some point Plato's sirens were replaced by Muses, possibly in the lost commentary on the Republic by the Neoplatonist Porphyry (c. 234–c. 305). A unique passage transmitting the "musical" version of Plato's image is found in the first book of Martianus Capella's fifth-century Neoplatonic treatise, De nuptiis Philologiae et Mercurii (On the marriage of Philology and Mercury). Martianus describes the Muses arriving at their celestial locations:

The upper planets and the sevenfold spheres produced together the clear harmonies of a certain sweet melody in a sound even more pleasant than usual, undoubtedly because they knew that the Muses were approaching. Passing through all the spheres one by one, each Muse stopped when she recognized the pitch that was familiar to her. Urania occupied the most distant sphere of the starry universe, which was carried along resonating an acute clear tone. Polymnia took the circle of Saturn; Euterpe that of Jupiter. Erato, once she had entered the sphere, sang the pitch of Mars. Melpone took the middle orbit where the sun makes the sky beautiful with his flaming light. Terpsichore was united with the gold of Venus. Calliope took possession of the sphere of Mercury, and Clio the innermost circle … on the moon, which resonated a deep pitch in a harsher tone. (Martianus Capella, De nuptiis, book 1, pp. 12–13)

Later in the treatise, Martianus describes Philology's traveling from earth through the heavenly bodies to the outermost sphere, giving the length of each leg of her journey in terms of a specific musical interval and indicating that the entire musical distance is equal to six whole tones, or an octave. Thus he presents an explicit working-out of the Platonic cosmological parallel between the ordering of the universe and the harmonic organization of a musical scale.