 Abū Bakr al-Rāzī (865–925 CE, 251–313 AH) was one of the greatest figures in the history of medicine in the Islamic tradition, and one of its most controversial philosophers. While we have ample surviving evidence for his medical thought, his philosophical ideas mostly have to be pieced together on the basis of reports found in other authors, who are often hostile to him. This concerns especially his notorious critique of religion, and his teaching that the cosmos is produced through the interaction of five “eternal principles”, namely God, Soul, matter, time, and place. We do however have direct access to one branch of his philosophy, namely ethics, represented by a work of advice called The Spiritual Medicine and a briefer rejoinder to his critics, The Philosophical Way of Life. (Note that Abū Bakr is not to be confused with other philosophers from the same city of Rayy, who are also called al-Rāzī: the most famous of these is Fakhr al-Dīn al-Rāzī, who wrote several centuries later.)