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Although over two dozen Arabic commentaries on the Canon of Medicine were composed between the twelfth and fourteenth centuries, historians of medicine have paid scant attention to them. Instead, these commentaries have often been dismissed as being uncritical expositions that further entrenched the dogma of Galenic/Avicennan medical theory. In this article, I shall show that in fact the opposite was the case for at least a subset of the Canon commentaries from this period. Fakhr al-Dīn al-Rāzī developed a new style of verification commentary across his philosophical corpus that he also deployed in his Canon commentary. Even though Fakhr al-Dīn largely adhered to Galenic/Avicennan medical theory, his commitment to verification (taḥqīq) led him to challenge and critically assess many facets of medical theory based on systematic, philosophical investigations. Ibn al-Nafīs, following in Fakhr al-Dīn’s footsteps, undertook a similar, systematic investigation into medical theory in his own Commentary on the Canon. However, in this case, verification led Ibn al-Nafis to challenge and modify several facets of medical theory. Moreover, as a trained, practicing physician, Ibn al-Nafīs also wanted to ensure that his commentary was useful for other practicing physicians. His commentary thus shows how a post-classical physician committed to the principles of verification and utility could employ philosophical argumentation, empirical observations and even occasional experiments to modify key aspects of Galenic/Avicennan medical theory and practice.