Abū Zayd al-Balkhī, Aḥmad b. Sahl (ca. 235–322/849–934), was a man of letters, theologian, philosopher and geographer, who also dabbled in various other sciences. The earliest sources of information on him are Ibn al-Nadīm's al-Fihrist and Yāqūt's Muʿjam al-udabāʾ, the latter being the most complete. Yāqūt, from whom subsequent sources derive all their information, derived his material from a book written by Abū Sahl Aḥmad b. ʿUbayd Allāh b. Aḥmad. Abū Sahl's own source was Abū Muḥammad Ḥasan b…