The Ikhwan al-Safa' (Brethren of Purity), the anonymous adepts of a tenth-century esoteric fraternity based in Basra and Baghdad, hold an eminent position in the history of science and philosophy in Islam due to the wide reception and assimilation of their monumental encyclopaedia, the Rasa'il Ikhwan al-Safa' (Epistles of the Brethren of Purity). This compendium contains fifty-two epistles offering synoptic accounts of the classical sciences and philosophies of the age; divided into four classificatory parts, it treats themes in mathematics, logic, natural philosophy, psychology, metaphysics, and theology, in addition to didactic fables. The Rasa'il constitutes a paradigmatic legacy in the canonization of philosophy and the sciences in mediaeval Islamic civilization, and has also shown a permeating influence in Western culture. This is the third volume in a series presenting the very first critical edition of the Rasa'il in its original Arabic, complete with the first fully annotated English translation. Epistle 5: On Music examines not just the technical, scientific, and mathematical aspects of music, but its cosmic, psychological, and spiritual dimensions. Music completed the classical training of the quadrivium (the sine qua non for further studies in philosophy and theology), and, as with other epistles of this group, there is much emphasis on numeric proportions and the underlying cosmic order of the universe. Technical concepts such as rhythm, tone, metre, and melody, along with the lute and its tunings as these relate to the fourfold Galenic theory, lead to a consideration of the psychological applications and the ultimately spiritual nature of music.