It is a popular fact that nine-tenths of the brain is not used and, like most popular facts, it is wrong. Not even the most stupid Creator would go to the trouble of making the human head carry around several pounds of unnecessary gray goo if its only real purpose was, for example, to serve as a delicacy for certain remote tribesmen in unexplored valleys.
Terry PratchettBut you ain't part of it, are you?" said Granny conversationally. "You try, but you always find yourself watchin' yourself watchin' people, eh? Never quite believin' anything? Thinkin' the wrong thoughts?
Terry Pratchett