This book introduces readers to Abū Bakr al-Rāzī (known as Rhazes in Latin), one of the most innovative and divisive figures of the early philosophical tradition in the Islamic world. It attempts to reconstruct his notorious theory of “five eternals” which posited four principles alongside God for the creation of the world, which led Razi to be charged with heresy by other authors. Central to the interpretation of this theory is Razi’s concern with theodicy, and his idea that Platonist physics is more fundamental than Aristotelian physics. Other topics discussed in depth include his critical reaction to contemporary Islamic theology, his medical works, his alchemical theories, his works on ethics, and his controversial views on religion and prophecy.