Medieval medicine in general, and the knowledge of Greek medicine that reached the Latin West through Avicenna in particular, have recently come to the forefront of scientific inquiry, as well as gained interest from a broader public, especially in relation to the study of botany and herbal medicine, where alternatives to conventional medical treatments are often sought. In contrast, the theoretical considerations underlying medieval herbal medicine have received significantly less attention. These can be succinctly captured in the introductory chapters of the second book of Avicenna's Canon of Medicine, which serve as a foundation for engaging with herbal medicine.