The linguistic system represented in the mind-brain gives some account of everything submitted to it: good sentences, bad sentences, sentences in other languages, ums and ahs, coughs; perhaps -Chomsky suggests- even to non-linguistic events like a squeaking door.

How is it that you can read this book? An obvious answer is because you know English. An equally obvious answer is because the light is on. These two explanations for an apparently trivial ability can illuminate a fundamental dichotomy: the difference between our knowledge of language and our use of that knowledge; between our competence and our performance. Your knowledge of grammar and vocabulary of English, your competence as a speaker of English, is prerequisite to your understanding this sentence; the exercise of this competence is made possible by the fact, among many others, that the light is on.