 The problem of knowledge is at the very heart of Crathorn’s thought. Going back to Roger Bacon, he held that the only way the human mind is able to know the external world is through species that resemble it (I Sent. q. 1, concl. 4). Bacon’s theory of the multiplication of species (multiplicatio specierum) was developed to explain causality using the model of optics. A thing c has an effect on another thing e through the multiplication of species s from c towards e, just as light is multiplied through the air when it illuminates an object. According to Bacon, the species s is a representative of c but with a lesser mode of being. Thus, the species is multiplied through the medium of air again and again until it arrives at e. The causal power of c is in this way conserved through the medium until it acts on e. The species is therefore both the cause (i.e., the same thing as the cause but with a diminished mode of being) and a likeness of that cause. Applying Bacon’s theory to epistemology, Crathorn asserts that we have no direct access to things in the external world and that we immediately perceive only their mental likenesses or representations, i.e., their species. These mental entities, whether we call them ‘species“ or ‘concepts” (the terms are equivalent for Crathorn), have the same nature as the things they resemble (ibid. concl. 8). Contrary to Bacon, the Dominican considers that the species has the same mode of being — i.e., material being — outside and inside the mind. As a consequence, all the species we have in the mind belong to the category of quality, for no real substance or quantity can exist materially in the human soul. Since concepts can only belong to the category of quality according to Crathorn, they must be mental qualities having the same nature as non-mental qualities and they must exist subjectively in the mind, which is to say that they exist in some part of the brain (Pasnau 1997). Crathorn writes, “the word ‘cognition’ (cognitio) stands for the idea (verbum) of the thing known, and that idea is the quality existing subjectively (subiective existens) in the mind or in some part of the brain” (I Sent. q. 1, concl. 1). Therefore, cognition is nothing but the species itself or the part of the soul where it is received and stored. From an ontological point of view, nothing distinguishes sensible cognition, imagination and intellection except the distinct part of the soul where the species is received. There is no difference between sensible and intelligible species.