The Western intellectual tradition has long recognized the importance of representation in thought. A dramatic example is provided by the philosopher René Descartes (1596–1650), who hypothesized the existence of an "evil demon" that deceived him in every way, causing all of his mental representations to be false. Descartes's emphasis on the role of mental representations in thought was influential, and subsequent thinkers—John Locke (1632–1704), David Hume (1711–1776), George Berkeley (1685–1753), and John Stuart Mill (1806–1873), to name a handful—expanded the discussion.
Contemporary work on mental representation owes as much to the theory of computation as it does to modern philosophy. In the mid-twentieth century, psychology was dominated by behaviorism, according to which mental entities, even if they exist, have no theoretical role in explanations of behavior. Instead, behavior was to be explained exclusively in terms of stimulus-response patterns. However, in the 1950s, opponents argued that this goal could not be achieved; one must posit internal representations. Moreover, these researchers came armed with a powerful new tool—the methods and concepts of the theory of computation.
The abstract principles of information processing upon which devices such as personal computers are based constitute the theory of computation. According to this theory, computation consists in the rule-governed manipulation of symbols. For example, consider doing long division using pencil and paper. A problem is written on the paper as a set of numerals, and solving it involves repeatedly applying a series of basic rules: division, multiplication, subtraction, writing additional symbols on the paper, and so forth. Analogously, an influential predecessor of today's computers—proposed in 1936 by the mathematician Alan Turing (1912–1954)—consists of a tape for storing symbols, a means for writing and reading symbols to and from the tape, and a controller encapsulating the rules for performing a calculation using those symbols.
Turing's machines are machines in name only; in actuality, they are abstract definitions, introduced to address a problem of mathematical logic. Regardless, he and others realized that variations on these machines could be physically realized. It was only a small step to infer that human cognitive processes are computational, involving the manipulation of mental representations according to "rules of thought."
Wielding the conceptual tools of computation, opponents of behaviorism initiated a flurry of research on mental representation, as well as creating entirely new areas of research, such as artificial intelligence and computational linguistics.
Three questions receiving considerable attention in contemporary research are: How do mental representations come to have their contents? What conditions must an entity satisfy in order to be a mental representation? And, what is the format of mental representation?