Allegory of the Cave
Mental images are an important topic in classical and modern philosophy, as they are central to the study of knowledge. In the "Republic", Book VII, Plato has Socrates present the Allegory of the Cave: a prisoner, bound and unable to move, sits with his back to a fire watching the shadows cast on the cave wall in front of him by people carrying objects behind his back. These people and the objects they carry are representations of real things in the world. Unenlightened man is like the prisoner, explains Socrates, a human being making mental images from the sense data that he experiences.