How does language work? How do children learn their mother tongue? Why do languages change over time, making Chaucer's English almost incomprehensible? Steven Pinker explains the profound mysteries of language by picking a deceptively simple single phenomenon and examining it from every angle. That phenomenon ' the existence of regular and irregular verbs ' connects an astonishing array of topics in the sciences and humanities: the history of languages; the illuminating errors of children as they begin to speak; the sources of the major themes in the history of Western philosophy; the latest techniques in identifying genes and imaging the living brain. Pinker makes sense of all of this with the help of a single, powerful idea: that language comprises a mental dictionary of memorized words and a mental grammar of creative rules.