One of the most influential books in the history of Arabic Botany and Medicine was Galen's "On the Powers [and Mixtures] of Simple Drugs (De Simplicium Medicamentorum)" which presented both his theory of pharmacology as well as a practical part that provided information on various simple drugs. Along with Dioscorides "Materia Medica" those texts were foundational to Arabic Botany and medicine. Powers of drugs expressed how they influenced the balance of the human body, and determining those powers became a central aspect in the formulation of the theory of mizāj/mixtures. But it also emerged as an essential theme in the overlap between medicine and philosophy. It embodied a set of epistemological questions that tested the boundaries of these disciplines and in many ways defined relations between them. The central question of this inquiry of How to determine the power of a simple drug was a question of scientific methodology that drew on the foundations of Epistemology: Reasoning (Qiyas) and Experience (Tajruba). In my paper, I examine how those epistemic categories were developed and expanded in the Arabic literature of Materia Medica during the 10th-11th Century through the works of several physicians and botanists, including Ibn Abi al- Ash'ath (d.c. 970), Ibn al-Jazzar (d. 979), Al-Majusi (d. 994) and Avicenna (d. 1037). I argue that those categories developed in innovative ways that borrowed from methodologies beyond Medicine and Natural philosophy.