Intelligence is most widely studied in humans, but has also been observed in animals and in plants. Artificial intelligence is the simulation of intelligence in machines.
Within the discipline of psychology, various approaches to human intelligence have been adopted. The psychometric approach is especially familiar to the general public, as well as being the most researched and by far the most widely used in practical settings.
Intelligence derives from the Latin verb intelligere, to comprehend or perceive. A form of this verb, intellectus, became the medieval technical term for understanding, and a translation for the Greek philosophical term nous. This term was however strongly linked to the metaphysical and cosmological theories of teleologicalscholasticism, including theories of the immortality of the soul, and the concept of the Active Intellect (also known as the Active Intelligence). This entire approach to the study of nature was strongly rejected by the early modern philosophers such as Francis Bacon, Thomas Hobbes, John Locke, and David Hume, all of whom preferred the word "understanding" in their English philosophical works. Hobbes for example, in his Latin De Corpore, used "intellectus intelligit" (translated in the English version as "the understanding understandeth") as a typical example of a logical absurdity. The term "intelligence" has therefore become less common in English language philosophy, but it has later been taken up (with the scholastic theories which it now implies) in more contemporary psychology.
The definition of intelligence is controversial. Some groups of psychologists have suggested the following definitions:
A very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings—"catching on," "making sense" of things, or "figuring out" what to do.
Individuals differ from one another in their ability to understand complex ideas, to adapt effectively to the environment, to learn from experience, to engage in various forms of reasoning, to overcome obstacles by taking thought. Although these individual differences can be substantial, they are never entirely consistent: a given person's intellectual performance will vary on different occasions, in different domains, as judged by different criteria. Concepts of "intelligence" are attempts to clarify and organize this complex set of phenomena. Although considerable clarity has been achieved in some areas, no such conceptualization has yet answered all the important questions, and none commands universal assent. Indeed, when two dozen prominent theorists were recently asked to define intelligence, they gave two dozen, somewhat different, definitions.
Besides those definitions, psychology and learning researchers also have suggested definitions of intelligence such as:
To my mind, a human intellectual competence must entail a set of skills of problem solving — enabling the individual to resolve genuine problems or difficulties that he or she encounters and, when appropriate, to create an effective product — and must also entail the potential for finding or creating problems — and thereby laying the groundwork for the acquisition of new knowledge.
The theory of Structural Cognitive Modifiability describes intelligence as "the unique propensity of human beings to change or modify the structure of their cognitive functioning to adapt to the changing demands of a life situation."
What is considered intelligent varies with culture. For example, when asked to sort, the Kpelle people take a functional approach. A Kpelle participant stated "the knife goes with the orange because it cuts it." When asked how a fool would sort, they sorted linguistically, putting the knife with other implements and the orange with other foods, which is the style considered intelligent in other cultures.
The common chimpanzee can use tools. This chimpanzee is using a stick to get food.
Although humans have been the primary focus of intelligence researchers, scientists have also attempted to investigate animal intelligence, or more broadly, animal cognition. These researchers are interested in studying both mental ability in a particular species, and comparing abilities between species. They study various measures of problem solving, as well as mathematical and language abilities. Some challenges in this area are defining intelligence so that it means the same thing across species (e.g. comparing intelligence between literate humans and illiterate animals), and then operationalizing a measure that accurately compares mental ability across different species and contexts.
It has been argued that plants should also be classified as being in some sense intelligent based on their ability to sense the environment and adjust their morphology, physiology and phenotype accordingly.
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Wolman, Benjamin B., ed. (1985). Handbook of Intelligence. consulting editors: Douglas K. Detterman, Alan S. Kaufman, Joseph D. Matarazzo. New York (NY): Wiley. ISBN978-0-471-89738-5. This handbook includes chapters by Paul B. Baltes, Ann E. Boehm, Thomas J. Bouchard, Jr., Nathan Brody, Valerie J. Cook, Roger A. Dixon, Gerald E. Gruen, J. P. Guilford, David O. Herman, John L. Horn, Lloyd G. Humphreys, George W. Hynd, Randy W. Kamphaus, Robert M. Kaplan, Alan S. Kaufman, Nadeen L. Kaufman, Deirdre A. Kramer, Roger T. Lennon, Michael Lewis, Joseph D. Matarazzo, Damian McShane, Mary N. Meeker, Kazuo Nihira, Thomas Oakland, Ronald Parmelee, Cecil R. Reynolds, Nancy L. Segal, Robert J. Sternberg, Margaret Wolan Sullivan, Steven G. Vandenberg, George P. Vogler, W. Grant Willis, Benjamin B. Wolman, James W. Soo-Sam, and Irla Lee Zimmerman.
Sternberg, Robert J.; Kaufman, Scott Barry, eds. (2011). The Cambridge Handbook of Intelligence. Cambridge: Cambridge University Press. ISBN9780521739115. Lay summary (22 July 2013). The Cambridge Handbook includes chapters by N. J. Mackintosh, Susana Urbina, John O. Willis, Ron Dumont, Alan S. Kaufman, Janet E. Davidson, Iris A. Kemp, Samuel D. Mandelman, Elena L. Grigorenko, Raymond S. Nickerson, Joseph F. Fagan, L. Todd Rose, Kurt Fischer, Christopher Hertzog, Robert M. Hodapp, Megan M. Griffin, Meghan M. Burke, Marisa H. Fisher, David Henry Feldman, Martha J. Morelock, Sally M. Reis, Joseph S. Renzulli, Diane F. Halpern, Anna S. Beninger, Carli A. Straight, Lisa A. Suzuki, Ellen L. Short, Christina S. Lee, Christine E. Daley, Anthony J. Onwuegbuzie, Thomas R. Zentall, Liane Gabora, Anne Russon, Richard J. Haier, Ted Nettelbeck, Andrew R. A. Conway, Sarah Getz, Brooke Macnamara, Pascale M. J. Engel de Abreu, David F. Lohman, Joni M. Lakin, Keith E. Stanovich, Richard F. West, Maggie E. Toplak, Scott Barry Kaufman, Ashok K. Goel, Jim Davies, Katie Davis, Joanna Christodoulou, Scott Seider, Howard Gardner, Robert J. Sternberg, John D. Mayer, Peter Salovey, David Caruso, Lillia Cherkasskiy, Richard K. Wagner, John F. Kihlstrom, Nancy Cantor, Soon Ang, Linn Van Dyne, Mei Ling Tan, Glenn Geher, Weihua Niu, Jillian Brass, James R. Flynn, Susan M. Barnett, Heiner Rindermann, Wendy M. Williams, Stephen J. Ceci, Ian J. Deary, G. David Batty, Colin DeYoung, Richard E. Mayer, Priyanka B. Carr, Carol S. Dweck, James C. Kaufman, Jonathan A. Plucker, Ursula M. Staudinger, Judith Glück, Phillip L. Ackerman, and Earl Hunt.