Since the realization, at the beginning of this century, that the treatise Speculum musicae had been incorrectly attributed to Jehan des Murs by its first editor, Edmond de Coussemaker, the actual author of this voluminous work of music theory from the early fourteenth century has remained a shadowy figure. The most certain detail of the author's identity is his name, contained within an acrostic spelled out over the initials that begin each of the seven books of the treatise, rendering the given name IACOBUS. The provenances of the three surviving manuscript sources, all dating from approximately a century after the proposed date of Speculum musicae, suggest an Italian bias to the transmission of the work, but, as physical documents, the manuscripts have yet to yield any clues to the author's origins. The treatise itself is a bit more helpful. Besides offering the author's name, clues within the text have allowed for the formulation of the following hypothesis concerning the career of Jacobus: that he was probably born in the diocese of Liège, that he was a student in Paris in the late thirteenth century, and that he returned to Liège to complete the final books of his treatise, Books 6 and 7 of Speculum musicae. In what follows, I will first briefly evaluate the evidence previously marshalled to support this hypothesis, and I will then discuss new information pertinent to the biography of the author.