Przegląd Tomistyczny, t. XXIX (2023), s. 123–150 ISSN 0860-0015 Robert Podkoński Center for the Philosophy of Nature, University of Lodz, Poland THE CONCEPT OF ‘VIRTUS/POTENTIA REI’ IN FOURTEENTH-CENTURY DISCUSSIONS ON TRANSMUTATION AND REACTION* In memory of Edith D. Sylla 1. Introduction — anachronistic understanding of terms and notion in the history of science. Te present paper has two purposes: a main, historical-philosophical one, and a secondary methodological one. My intent is to reconstruct the understanding of the concepts ‘virtus’ and ‘potentia rei’, that were frequently invoked and used interchangeably in fourteenth-century commentaries on Aristotle’s “De gener- atione et corruptione”, mainly in the context of discussions on transmutation and the reaction(s) of the elements. Te secondary purpose of this article — on which I focus frst — is to turn the reader’s attention to a specifc inclination that characterizes, as it seems, the majority of historians of philosophy and sci- ence. Tanks to the awaressness of this inclination, in fact, I noticed a frequent improper understanding of the above-mentioned Latin terms in the secondary literature of the topic. What I have in mind here is the tendency towards an anachronistic under- standing of ancient and medieval scientifc texts in general, and the concepts employed within them in particular, as if these concepts were understood and/or defned exactly in the same manner centuries ago as they are in modern times. On the one hand, this tendency is rooted in the fact that it is obviously easier — * Te present paper is an efect of the research funded by the National Science Centre (Naro- dowe Centrum Nauki, NCN) project NCN „Opus 19” nr 2020/37/B/HS1/00105.