The surviving texts of medieval treatises, together with their 'metadata' (prohemiums, explicits, marginalia, etc.) are often the richest sources of information about their authors. Unfortunately, the 'Jacobus' who completed a monumental Speculum musicae in the 1320s is remarkably cagey about his identity, limiting himself to indicating in the Prohemium that his first name may be obtained by forming an acrostic from the first letters of each of the seven books that comprise the treatise. Alas, for a long time readers did not connect the litterae (albeit with at least one important exception cited below) and misread an explicit in the only complete manuscript copy of the Speculum, with the result that the treatise was attributed to Jacobus's contemporary Johannes de Muris from the sixteenth to the twentieth centuries.
It was only in the 1920s that the acrostic was finally resolved, pointing to 'Jacobus' as the author of the treatise. Passing references to liégeois chant melodies in the tonary of Book VI were taken to imply that Jacobus spent the final years of his life in Liège. This speculation led Roger Bragard to formulate a new one in the 1950s—namely, that because it was relatively common at that time to retire in one's place of origin, Liège was also the likely city of Jacobus's birth. Since then, the Speculum musicae has been generally ascribed to a 'Jacques de Liège' with more or less conviction, perhaps due to the lack of credible alternatives rather than to the strength of Bragard's argument.
Naturally, all these theories, clues, and speculations about the identity of the author of the Speculum undergo close scrutiny in the initial chapters of Margaret Bent's new monograph on the subject. In the initial chapter, Bent swiftly puts Bragard's argument to rest by showing that his theorem was built on circumstantial data and speculative arguments (pp. 14–19): the references to liégeois musical practices in the Speculum do not necessarily place Jacobus in that city at any time in his career, and the observation that it was relatively common in pre-modern times to retire to one's places of origin cannot be invoked to make inferences on individual cases (p. 15). In due course, Bent also rules out other candidates, regarding them as unlikely to be our 'Jacobus' for biographical or music-theoretical reasons (pp. 47–53; 72–80).
The main thesis of the book, fully articulated in chapters 4–9, originates from an entry in a 1457 inventory from Vicenza Cathedral that mentions 'a book containing the musica of Master Jacobus de Ispania, divided into seven books, of which the first letters make this name, Jacobus'; this is presumably the same volume that a local canon had bequeathed to the church in 1419 (pp. 63–4). According to Bent, there is only one known historical figure who matches the description 'Magister Jacobus de Ispania' and who could also have authored the Speculum. That person is 'James of Spain', usually believed to be an illegitimate son of King Alfonso X ('The Wise'), half-brother of Eleanor of Castile, queen consort of Edward I, King of England. Bent, however, deftly argues that James's father was instead Alfonso's younger brother, the Infante Enrique (pp. 82–4; 106–8). Eleanor raised James at court, and partly because of the boy's illegitimate birth she steered him towards a life of learning and scholarship, for which he showed much promise from his early years, rather than towards the political or military career that would have been a natural destination for a man of his status (p. 106). Needless to say, James was in a position to provide more than adequately for his material needs through a long string of often incompatible benefices and prebends, despite—here is another surprise—never taking holy orders. In short, Bent's 'James of Spain' hypothesis fits the few biographical details that may be plausibly inferred about the author of...