In QQ.23–31 of Olivi’s Quaestiones in secundum librum Sententiarum (Summa II) and in Bacon’s De multiplicatione specierum (DMS) 1.3, we find an intriguing discussion concerning the link between agent and patient in accounts of physical action in the Aristotelian tradition. Both thinkers hold that species were the link between agent and patient; they disagree, however, about the definition and function of species. The dispute leads the two thinkers to develop and clarify their accounts of physical action. They discuss temporality, secondary causality, active potentiality, and the distinction between virtual and substantial contact. This paper provides an account of Olivi’s theory of species in medio and clarifies how it differs from Bacon’s theory. It throws a spotlight on a significant episode in the history of philosophy, in which Aristotelian concepts were found unsuitable to account for action at a distance and in the interior of the patient, and hence new concepts of virtual action and species had to be devised.