In Search of Jacobus Although hitherto known as Jacobus Leodiensis, Jacques de Liège, Jacobus de Montibus, the identity of the author of the longest, most systematically organised, and most learned music treatise of the Middle Ages is still unresolved. Some earlier hypotheses acquired the status of fact simply from the absence of competing evidence. The only thing we knew with any certainty about the author of the Speculum musicae was that, as he tells us at the end of the proemium, the initial letters of the seven books spell his name:

JACOBUS. Nothing more, no colophons, no physical clues in the surviving manuscripts.1 In his 1924 Freiburg dissertation Walter Grossmann noted the acrostic but did not solve it; he evidently worked only from a partial photographic copy and did not have access to the entire manuscript.2 Heinrich Besseler was the first to report, in 1925, his teacher Willibald Gurlitt’s solution of that authorial acrostic.3 This overturned its unwittingly ironic misattribution since the sixteenth century to another

polymath, Johannes de Muris, famed as a mathematician and astronomer as well, a primary but unnamed object of the author’s attacks;4 the attribution of this enormous work augmented his reputation. This is ironical, as the Speculum is the only extant treatise to set out clear differences between the ars antiqua and the ars nova, and in favour of the former. The error derived from an original attribution at the end of the only complete source of the Speculum, the fifteenth-century manuscript Paris, BnF lat. 7207 (‘Explicit tractatus Musice Magistri Johannis de Muris’). This attribution, in the same hand as the entire volume, was taken to apply to the whole contents and not just the final five folios (ff. 294r-299v) containing the ‘B’ version of Muris’s digest of Boethius’s De institutione musica, the so-called Musica speculativa (‘Quoniam musica est de sono relato ad numeros’). Muris’s text follows the earlier and less conspicuous anonymous explicit of the Speculum (on f. 293r).5