During the Middle Ages Aristotle's treatises were accessible to intellectuals via translations and commentaries. Among his works on natural philosophy, the zoological books received relatively little scholarly attention, though several medieval commentators carefully studied Aristotle's investigations of the animal kingdom. Averroes completed in 1169 a commentary on an Arabic translation of Aristotle's Parts of Animals and Generation of Animals. In 1323 Gersonides completed his supercommentary on a Hebrew translation of Averroes' commentary. This article examines how these two medieval commentators interpret (...) the first book of Aristotle's Parts of Animals, at the center of which stand methodological questions regarding the study of animals. Aristotle's discussion of classification is presented by Averroes and Gersonides in light of an epistemological debate concerning the requisite method for scientific inquiries and discoveries. Sense perception is contrasted with rational reasoning, and ultimately a combined method is proposed, sense perception maintaining supremacy. These commentators outline a clear link between the systematic arrangement of animal species as offered by Aristotle, and his subsequent logical demonstrations which, according to them, form the core of biological investigations. (shrink)