The term “alchemy,” born out of early modern professional polemics among chemists, is problematic as a historical category. The present article shifts away from asking what pre-modern alchemy “really” was, to asking how medieval scholars writing in Greek and Arabic thought about the practice of treating and combining naturally occurring substances to produce apparently quite different substances, and how they interpreted, valorized, or critiqued this practice and its results — in other words, what they thought about chemistry.