Emanating from a cosmos ordered according to Pythagorean and Neoplatonic principles, the Boethian musica mundana is the type of music that ‘is discernible especially in those things which are observed in heaven itself or in the combination of elements or the diversity of seasons’. At the core of this recurring medieval topos stands ‘a fixed sequence of modulation [that] cannot be separated from this celestial revolution’, one most often rendered in medieval writings as the ‘music of the spheres’ (musica spherarum). In the Pythagorean and Neoplatonic cosmological traditions, long established by the time Boethius wrote his De institutione musica, the music of the spheres is just one possible manifestation of the concept of world harmony. It pertains to a universe in which musical and cosmic structures express the same mathematical ratios, each of the planets produces a distinctive sound in its revolution and the combination of these sounds themselves most often forms a well-defined musical scale. Although the Neoplatonic world harmony continued to function in medieval cosmology as the fundamental conceptual premise, the notion of the music of the spheres, despite its popularity among medieval writers, was generally treated neither at any significant length nor in an innovative fashion. Quite exceptional in this respect is the treatise that forms the subject of the present study, a text beginning Desiderio tuo fili carissime gratuito condescenderem and attributed to an anonymous bishop in the late thirteenth-century manuscript miscellany now in the Biblioteca Apostolica Vaticana (Barb. lat. 283, fols. 37r-42v) but probably coming from a Franciscan convent in Siena. This seldom considered work affords a remarkable and special insight into the ways in which old and new ideas converged, intermingled and coexisted in the dynamic and sometimes volatile cross-currents of medieval scholarship.