"...when Dante insists on describing the quality of the musical performances of damned, penitent and blessed souls, he does not do so out of mere aesthetic interest. Music is an agent of divine grace and its use is consistent: Dante employs music in Paradiso as a rain to bless the souls and as a mystical means of expression in order to circumvent the engulfment of poetic language, while in Purgatorio plainchant provides a means for purging sins. By the same token, music in Inferno mocks the damned and reminds them of the salvation they will never reach, in a consistent parody of sacred music. Ironically, the same tool is therefore employed both to fulfill the desire of eternal happiness and to frustrate it.
Dante introduces into his poem the different styles of music of his time, showing an impressive knowledge, if not of the compositional techniques, at least of the repertory and its liturgical uses. He makes music the language of the spirit and incorporates this art into the monumental construction of his other world. References to polyphony are neither accidental nor decorative, but constitute a complex architecture, whose inherently musical meaning mirrors the reconciliation of multiplicity and unity. It is Dante's solution to the age-old problem, of reconciling the multiplicity of individuals with the unity of the Creator. The chants of the Commedia are therefore not a mere accompaniment to the pilgrim's voyage, but an essential component of it. Harmony, in a political, spiritual and musical sense, becomes the end of Dante's journey to polyphony.
The transition from monophony to polyphony accompanies a cathartic progress toward the spiritual union with the Creator. There is a specifically musical quality to this purification process: for the penitents, monophonic singing is the tool of purification as the individual struggles for harmony. The songs in Purgatorio therefore constitute a pharmakon, a remedy to heal the soul from sin, which seems to revive the Pythagorean notion of music as medicine. The change to polyphony in Paradiso reflects the harmony with the Supreme Being, realized, spiritually as well as musically, through the simultaneous resonance of the souls' melodies within the music of God."