Mental representations are among the most mysterious entities involved in cognition. How is the brain's model of the world formed, how is it used, how is it corrected, how essential is it to our knowledge of the world? Psychologists have been trying to comprehend the nature of these internal mechanisms by models of their own, which fall into three main categories: computational, biological, and constructivist. But whether these explanations draw on the experience of computer programming for artificial intelligence, or on an analysis of the interrelations between neurons in the brain, or on some mechanical or abstract analogy, the purpose is always to find the metaphor which is most useful in scientific and predictive terms. All these approaches are represented in Knowledge and Representation (originally published in 1982), which is designed to convey the full richness and variety of research at the time and theorizing in this area of cognitive psychology.