  William was one of the first thinkers in the Latin West to begin to engage seriously with Aristotle's writings on metaphysics and natural philosophy and with the thought of Islamic and Jewish thinkers, especially Avicenna (Ibn Sīnā, 980–1037) and Avicebron (Solomon Ibn Gabirol, 1021/2–1057.8). The works of these thinkers, which had been coming into circulation in Latin translations from around the middle of the twelfth century, presented both philosophical sophistication and theological danger. Indeed, in 1210, public and private lecturing on Aristotle's works on natural philosophy and commentaries on them was forbidden at Paris, and in 1215 the papal legate, Cardinal Robert of Courçon forbade “masters in arts from ‘reading,’ i.e., lecturing, on Aristotle's books on natural philosophy along with the Metaphysics and Summae of the same (probably certain works of Avicenna and perhaps of Alfarabi)” (Wippel 2003, 66). This prohibition remained in effect until it unofficially lapsed around 1231. Nevertheless, it must be emphasized that personal study of these texts had not been forbidden, and William clearly worked hard to master them. He came to be keenly aware of the incompatibility of much of the teaching of these works with Christian doctrine, yet at the same time he found them to be a source of philosophical inspiration, and his thought brims with ideas drawn from Avicenna and Avicebron. His attitude to these thinkers is expressed well in a passage in On the Soul, where, regarding Aristotle, he writes that: