The Speculum musicae of Magister Jacobus de Ispania from Liège is the ultimate Medieval Summa of music. Compiled probably during the 1330s, it comprises no fewer than seven books, altogether totaling more than 375,000 words. Princeton musicologist Rob C. Wegman offers a translation of the seventh and final book, which deals with contemporary polyphony. This part of the Speculum is the notorious-and uncommonly impassioned-diatribe against new musical and notational practices that had gained currency in France in the second quarter of the fourteenth century. Jacobus proves himself thoroughly schooled in Scholastic philosophy and prosecutes the case with relentless determination, using his consummate rhetorical skills and his fierce critical intelligence to full advantage. What drove him to launch the attack was his sense of personal loyalty to the music and musicians he had loved during his years as a university student at Paris, probably in the 1290s, as well as his faith in the decisive power of rigorous and methodical reasoning. Yet to his infinite sadness, his demonstrations proved of little avail against the more powerful contemporary forces of changing musical taste and practical expediency. Magister Jacobus concludes his treatise with a moving prayer of thanksgiving, in which he looks forward to the life to come, and appears ready to part from this world, which had so bitterly disappointed him in his final years.