The Esse Similitudinale of Species in Roger Bacon’s Liber de sensu et sensato

Roger Bacon’s theory of perception, grounded in his original conception of species and of their mul-tiplication and reception, has always been the object of considerable scholarly attention. Yet, modern studies have traditionally focused on Bacon’s mature works. In these works, especially in the De multiplicationespecierum, Bacon endorses a strong “materialist” position with respect to the kind of being that species have in the medium and in the senses. In this paper I show that an alternative conception of the ontology of species is presented by Bacon in his commentary on Aristotle’s De sensu et sensato. This work is situated at a critical juncture in Bacon’s intellectual journey, since it is the last of his extant Aristotelian commentaries, while it precedes all his mature works. As I will argue in this paper, in these works Bacon assigns to species an ‘esse similitudinale’, which he explic-itly defines as the composition of esse materiale and esse formale. As a result, Bacon did not always deem species to be merely composed of esse materiale. I end by suggesting that this finding might have some implications on the interpretation of Bacon’s mature ontology of species as detailed in the De multiplicatione specierum.