Islamic physicians made many advances in medicine during the Golden Age, building upon Greek knowledge and adding their own discoveries. Central to Islamic medicine was the belief that Muslims had a duty to care for the sick, based on verses from the Quran and sayings of the Prophet Muhammad. Notable Islamic scholars who contributed greatly to medicine included al-Razi, who wrote extensively on physiology; Ibn Sina (Avicenna), author of the Canon of Medicine; and al-Zahrawi, a renowned surgeon whose works influenced European medicine. The Islamic world also saw advances in optics thanks to the work of scientists like al-Hazen.