critics of scientific realism ask 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 critics, scientific realists cannot explain where the images and their perceiver exist in the brain. to use the analogy of the computer screen, these critics argue that cognitive science and psychology have been unsuccessful in identifying either the component in the brain (i.e., "hardware") or the mental processes that store these images (i.e. "software").