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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 uses the metaphor of a prisoner in a cave, bound and unable to move, sitting with his back to a fire and watching the shadows cast on the wall in front of him by people carrying objects behind his back. The objects that they are carrying are representations of real things in the world. The prisoner, explains Socrates, is like a human being making mental images from the sense data that he experiences.
More recently, Bishop Berkeley's proposed similar ideas in his theory of idealism. Berkeley stated that reality is equivalent to mental images — our mental images are not a copy of another material reality, but that reality itself. Berkeley, however, sharply distinguished between the images that he considered to constitute the external world, and the images of individual imagination. According to Berkeley, only the latter are considered "mental imagery" in the contemporary sense of the term.
The eighteenth century British writer, Dr. Samuel Johnson, criticized idealism. When asked what he thought about idealism (while out on a walk in Scotland) he is alleged to have replied "I refute it thus!" as he kicked a large rock and his leg rebounded. His point was that the idea that the rock was just another mental image and had no material existence of its own, was a poor explanation of the painful sense data he had just experienced.
David Deutsch addresses Johnson's objection to idealism in The Fabric of Reality[How to reference and link to summary or text] when he states that if we judge the value of our mental images of the world by the quality and quantity of the sense data that they can explain, then the most valuable mental image — or theory — that we currently have is that the world has a real independent existence and that humans have successfully evolved by building up and adapting patterns of mental images to explain it. This is an important idea in scientific thought.
Critics of scientific realism ask[How to reference and link to summary or text] how the inner perception of mental images actually occurs. This is sometimes called the "homunculus problem" (see also the mind's eye). The problem is similar to asking how the images you see on a computer screen exist in the memory of the computer. To scientific materialism, mental images and the perception of them must be brain-states. According to these philosophers, scientific realists cannot explain where the images and the perceiver of them exist in the brain or its functions. To use the analogy of the computer screen, these critics argue that cognitive science and psychology has been unsuccessful in identifying the component in the brain (e.g. 'hardware' such as a computer graphics card) or the mental processes that store these images (e.g. 'software' such as a graphics device driver).