It is well-known that Abū Bakr Muḥammad ibn Zakarīyā al-Rāzī (d. ca. 925) wrote in several medical genres, including encyclopaedia, case notes, aphorisms, Doubts about Galen, and self-help. Yet, al-Rāzī is not well-known as a medical commentator, despite the fact that medical commentary was an important genre in late antiquity as well as the medieval Islamic world. In a pathbreaking article, Franz Rosenthal drew attention to the fact that Arabic commentaries on the Hippocratic Aphorisms preserved valuable fragments of al-Rāzī’s lost commentary on the Aphorisms. During a project to survey the entire Arabic commentary tradition on the Aphorisms, we have found over 80 fragments and quotations attributed to al-Rāzī. Some of these come from al-Rāzī’s Doubts about Galen, but the majority appear to stem from his lost commentary on the Aphorisms. This talk has a two-fold aim: to raise awareness about this lost text and give some preliminary findings about its nature, structure and historical significance; and to discuss two examples dealing with medical epistemology to illustrate a side of Rāzī’s scientific output that is virtually unknown.