Pinker attempts to trace the outlines of the language instinct by citing his own studies of language acquisition in children, and the works of many other linguists and psychologists in multiple fields, as well as numerous examples from popular culture. He notes, for instance, that specific types of brain damage cause specific impairments of language such as Broca's aphasia or Wernicke's aphasia, that specific types of grammatical construction are especially hard to understand, and that there seems to be a critical period in childhood for language development just as there is a critical period for vision development in cats. Much of the book refers to Chomsky's concept of a universal grammar, a meta-grammar into which all human languages fit. Pinker explains that a universal grammar represents specific structures in the human brain that recognize the general rules of other humans' speech, such as whether the local language places adjectives before or after nouns, and begin a specialized and very rapid learning process not explainable as reasoning from first principles or pure logic. This learning machinery exists only during a specific critical period of childhood and is then disassembled for thrift, freeing resources in an energy-hungry brain.