A term coined by British mathematician Ian Stewart and biologist Jack Cohen in their 1997 book Figments of Reality, defined as the cultural capital that is available to us in the form of external media (eg legends, folklore, books, music or video tapes, CDs, DVDs, etc).
Stewart and Cohen contrast extelligence with intelligence, or the knowledge and cognitive processes within the brain. However, they claim that there is an interdependent relationship between knowledge-inside-one’s-head (intelligence) and knowledge-outside-one’s-head that can be readily accessed (extelligence) and regard this “complicity” (a combination of complexity and simplicity) as fundamental to the development of consciousness in evolutionary terms for both the species and the individual. Through the idea of complicity, Cohen and Stewart relate the individual to the sum of human knowledge.
Although Cohen’s and Stewart’s respective disciplines are biology and mathematics, their description of the complicity of intelligence and extelligence is in the tradition of psychology theorists Jean Piaget, Belinda Dewar and David A Kolb. Philosophers, notably Karl Popper, have also considered the relation between subjective knowledge (which he calls world 2), objective knowledge (world 1) and the knowledge represented by man-made artefacts (world 3).