 Incredibly, Crathorn affirms that whenever one is thinking of a white thing, the mind of that person actually becomes white. His notion of similarity is strict and uncompromising. Mental concepts cannot resemble substances but only qualities of substances (I Sent., q. 1, concl. 7) because the species of substance would have to be a substance itself and our minds would turn into a new substance if we thought of it. It also cannot be a pure quantity because in thinking of infinite magnitudes, our minds would become infinite, and the same is true for the other categories besides quality. According to Crathorn, our ability to conceptualize is therefore limited to natural concepts of qualities, which in being conceived become qualities of the soul.