The Decisive Treatise is perhaps the most controversial work of Averroes (Ibn Rushd, 1126-1198) and belongs to a trilogy which boldly represent the philosophical contribution to Islamic theology of this famous Andalusian commentator on Aristotle. The Decisive Treatise is a fatwa (a legal opinion) that the judge, Averroes, promulgated for his fellow Malikite jurists in order to demonstrate that the study of philosophy is not only licit from the point of view of religious law, but even mandatory for the skilled people. However, many subjects are dealt with in this comparatively short book: An epistemology aimed to show that philosophical truth and religious truth are not in contradiction; a sociology of knowledge pointing out that humans are classified in three classes (philosophers, theologians, common folk); a Qur'anic hermeneutics suggesting how to approach philosophically the Holy Book in agreement with religious requirements and linguistic rules.