An (Apparent) Exception in the Aristotelian Natural Philosophy: Antiperistasis as Action on Contrary Qualities and its Interpretation in the Medieval Philosophical and Medical Commentary Tradition

This paper explores the scholastic debate about antiperistasis, a mechanism in Aristotle’s dynamics described in the first book of Meteorology as an intensification of a quality caused by the action of the contrary one. After having distinguished this process from a homonymous, but totally different, principle concerning the dynamics of fluids that Aristotle describes in his Physics, I focus on the medieval reception of the former. Scholastic commentators oriented their exegetical effort in elaborating a consistent explanation of an apparently paradoxical process like the intensification of a quality by the opposite one. From the fourteenth century onwards, most of the commentators resorted to the theory of the multiplication of species, according to which each entity acts through the emission of simulacra of the objects (species) that spread spherically in the medium. When these rays encounter an obstacle, such as a contrary quality, they are pushed back towards their source. The reflection of the species determined by the surrounding and opposite quality produces a concentration of the first one, which is therefore intensified. Another distinctive feature of the scholastic interpretation of Aristotle’s antiperistasis is the convergence between the discussions on inorganic and organic matter, physical and medical discourse. This convergence found its most significant expression in the adoption of the model described in the first book of Aristotle’s Meteorology to the biological context of Hippocrates’s Aphorisms I, 15. Following Galen’s exegesis of this passage, medieval commentators established a link between physics and medicine substantially extraneous to Aristotle’s theory.