In addition to the works of Aristotle, as they were known in the Middle Ages, medieval students read such books as Porphyry's Isagoge, or Introduction to Aristotle; the criticism of Aristotle's Categories, by Gilbert de la Porrée, known as the Sex Principia; the Summulae Logicales, a semi-grammatical, semi-logical treatise by Petrus Hispanus (Pope John XXI.); the Parva Logicalia of Marsilius of Inghen; the Labyrinthus and Grecismus of Eberhard; the Scriptural commentaries of Nicolaus de Lyra; the Tractatus de Sphaera, an astronomical work by a thirteenth-century
