The article Is language the key to human intelligence? , written by David Premack, professor of psychology at the University of Pennsylvania, explains how humans have displayed their intelligence through language, unlike animals whose language (any) has not evolved at all. Premack uses examples such as the grammar and syntax of human language and explains the uniqueness and evolution of language over time. He states that humans have a system of six symbols: "two that we evolved - the genetic code and spoken language - and four that we invented: written language, Arabic numerals, musical notation, and laboratory notation (a system for coding the choreography)". One word he emphasizes from time to time is Recursion ("causes the words of a sentence to be widely separated and yet dependent on each other"); arguing that humans have learned both recursive and non-recursive grammar, while tamarin monkeys have failed to learn a recursive grammar. This experiment, he believes, could help explain why language in animals did not evolve over a period of time. Premack also looked at other factors...