Until the twentieth century, deductive logic and the psychology of human thought were considered to be the same topic. The mathematician George Boole titled his 1854 book on logical calculus An Investigation of the Laws of Human Thought. This book was designed "to investigate the fundamental laws of those operations of the mind by which reasoning is performed." Humans did not always seem to operate according to the prescriptions of logic, but such lapses were seen as the malfunctioning of the mental machinery. When the mental machinery functioned properly, humans were logical. Indeed, it is human rationality, the ability to think logically, that for many thinkers throughout time has separated humans from other animals (for example, Aristotle's man as rational animal) and defined the human essence (for example, René Descartes's "I think, therefore I am").
As a quintessential mental process, the study of reasoning is an integral part of modern cognitive psychology. In the mid-twentieth century, however, when psychology was in the grip of the behaviorist movement, little attention was given to such "mentalistic" conceptions, with the exception of isolated works such as Frederic C. Bartlett's studies of memory and Jerome S. Bruner, Jacqueline J. Goodnow, and George A. Austin's landmark publication A Study of Thinking (1956), dealing with, among other topics, induction and concept formation. The development of the digital computer and the subsequent application of the computer as a metaphor for the human mind suggested new methods and vocabularies for investigating mental processes such as reasoning, and with the ascendancy of the cognitive approach within experimental psychology and the emergence of cognitive science, research on human reasoning has become central in attempts both to understand the human mind and to build machines that are capable of independent, intelligent action.
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