Just as the senses constantly receive impressions, so too simulacra of things 
impact the mind. The simulacra that affect the mind are subtler and far thinner 
than the sense-producing images; because they are like a spider's web or leaves of gold, these images easily cling together in the air and produce strange images in the mind. 
Thus, the mind perceives mixed or distorted images of things that never existed in reality—for example, mental visions of Centaurs, monsters, and ghosts. The images seen in dreams, too, are the result of these flitting simulacra perceived by the mind.