Mental images are an important topic in classical and modern philosophy

Have you ever wondered why you seem to get a mental picture of something happening when you are reading a book? Or maybe when you have a daydream? These images that one experiences appear to be like pictures in your head. For example, when a musician hears a song they can sometimes "see" the song notes in their head. This is different from an after image. For example, after-image from an event that is induced is considered not under our conscious control. By contrast, however, when we call up an image in our imagination or minds, it is considered to be voluntary. Therefore, we can characterize our imagery as being various degrees of our conscious control.

According to some biologistsour experiences of the world around us are stored as mental images which we can then associate and compare with other mental images and we can synthesize completely new images, for example when we dream or imagine. This theory states that this process allows us to form useful theories of how the world works based on likely sequences of mental images, without having to directly experience that outcome, for example through the processes of deduction or simulation. Whether other creatures have this capability is debated.
Philosophical Ideas

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 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 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 the inner perception of mental images actually occurs. This is sometimes called the "homunculus problem" . 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 or the mental processes that store these images.