Averroës (Ibn Rushd, 1126-1198) emerged from an eminent family in Muslim Spain to become the first and last great Aristotelian of the classical Islamic world; his meticulous commentaries influenced Christian thinkers and earned him favorable mention (and a relatively pleasant fate) in Dante’s Divina Commedia. The Book of the Decisive Treatise was and remains one his most important works and one of history’s best defenses of the legitimate role of reason in a community of faith. The text presents itself as a plea before a tribunal in which the divinely revealed Law of Islam is the sole authority; Averroës, critical of the anti-philosophical tone of the Islamic establishment, argues that the Law not only permits but also mandates the study of philosophy and syllogistic or logical reasoning, defending earlier Muslim philosophers and dismissing criticisms of them as more harmful to the Islamic community than the philosophers’ own views had been. As he details the three fundamental methods the Law uses to aid people of varied capacities and temperaments, Averroës reveals a carefully formed and remarkably argued conception of the boundaries and uses of faith and reason.