  Both Olivi and Ockham seem to take it for granted that species are representations and hence a thing added to the object and the cognizer. There were, however, two versions of the species theory of cognition in the thirteenth century. The most influential one was foremost associated with Roger Bacon. According to him, species are representations, that is, real extended images like objects representing the thing cognized to the cognizer. The other version of the theory was defended by Aquinas. According to him, species were not representations but the forms themselves of the objects cognized under a different mode of being. The species were not real but spiritual and as such there are no physical change of for example the eye in a visual perception; only a spiritual one.