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 t