The Speculum musicae of the early fourteenth century is by a long way the largest medieval treatise on music. For nearly a century, its author has been known as Jacques de Liège or Jacobus Leodiensis. ?Jacobus? is certain, fixed by an acrostic declared within the text; Liège is hypothetical, based on evidence shown here to be less than secure. New documentary evidence identifies its author as ?Magister Jacobus de Ispania?. It is here suggested that the author of the Speculum is either someone who left no paper trail or James of Spain, a nephew of Eleanor of Castile, whose career is documented mostly in England. The book traces his career and the likelihood of his authorship of the Speculum musicae.