De Musica, by Saint Augustine of Hippo, 387-391.
Augustine’s treatise De Musica discusses music as a liberal art. The treatise, divided into six books, focuses on the musical theoretical aspects of metrics and rhythmics. The first five books were written prior to his conversion to Christianity. The treatise does not discuss music performance but, rather, music as a speculative science. Augustine was concerned with the philosophical and mathematical principles integral to enjoying music. His belief, as was common during the time, was that music affected the soul and, thus, could influence and determine the ethics, actions and morals of humans.
English version available through Columbia University Libraries:
Barnard Library Stacks, call number: BR65A52 E6 1947 vol.2
Attribution: Thesaurus Musicarum Latinarum, School of Music, Indiana University, Bloomington, IN 47405