Criticism of authority was a prominent feature of medieval philosophical writing. In this study the critiques of two contemporaneous scholars, Moses Maimonides and Fakhr al-Dīn al-Rāzī, are compared. Maimonides criticized Hellenistic authorities, mainly Aristotle. However, the starting point for his critique was Aristotle's admission of the limitations of his own inquiries. Maimonides admired Aristotle's questioning of his own conclusions; indeed, his own thought was characterized by constant self-doubt. Al-Rāzī criticized an earlier Muslim scholar, Ibn Sīnā (Avicenna), an intellectual giant whose imprint was strongly felt in philosophy and medicine. Al Rāzī used his commentaries on a number of Ibn Sīnā's books as a stage for criticizing the master and for arguing for his own, alternative viewpoints.