 These different epistemological explanations depend also on differences in ontology. According to medieval Realists, individuals cannot be the proper (not even a proper) and adequate object of knowledge for any intellect, since (1) their world includes common natures or essences besides individual things; (2) individuals stem from species by means of prime matter (plus quantity); and (3) prime matter is per se something opaque and unintelligible. Apart from the unsuitable theological consequences (how is it possible that God knows human beings perfectly, if every kind of individual is per se something not completely intelligible?), the realist account entails a paradox, as the phantasm already enables us to grasp the substance and the accidental properties of an individual thing far before the conversion to itself which follows intellective cognition, because it is an appropriate image (or representation) of the particular material object perceived by the senses. On the other hand, the punctum dolens of any nominalist description of the process of knowledge lies in the difficulty in explaining the nature and origin of abstractive cognition and the relation which holds between the universal sign and the individuals which it refers to—how can they constitute a natural set? How can we know under a universal form (universaliter) something which is in itself absolutely singular?