A third idea: An infinite range of behavior can be generated by finite combinatorial programs in the mind. […] The clearest example is the Chomskyan revolution in language. (15) Language is the epitome of creative and variable behavior. Most utterances are brand-new combinations of words, never before uttered in the history of humankind. We are nothing like Tickle Me Elmo dolls who have a fixed list of verbal responses hard-wired in. But, Chomsky pointed out, for all its openendedness language is not a free-for-all; it obeys rules and patterns. An English speaker can utter unprecedented strings of words such as Every day new universes come into existence, or He likes his toast with cream cheese and ketchup, or My car has been eaten by wolverines. But no one would say Car my been eaten has wolverines by or most of the other possible orderings of English words. Something in the head must be capable of generating not just any combination of words but highly systematic ones.

That something is a kind of software, a generative grammar that can crank out new arrangements of words. A battery of rules such as “An English sentence contains a subject and a predicate,” “A predicate contains a verb, an object, and complement,” and “The subject of eat is the eater,” can explain the boundless creativity of a human talker. With a few thousand nouns that can fill the subjective slot and a few thousand verbs that can fill the predicate slot, one already has several million ways to open a sentence. The possible combinations quickly multiply out to unimaginable large numbers. Indeed, the repertoire of sentences is theoretically infinite, because the rules of language use a trick called recursion. A recursive rule allows a phrase to contain an example of itself, as in She thinks that he think that they thinki that he knows and so on, ad infinitum. And if the number of sentences is infinite, the number of possible thoughts and intentions is infinite too, because virtually every sentence expresses a different thought or intention. The combinatorial grammar for language meshes with other combinatorial programs in the head for thoughts and intentions. A fixed collection of machinery in the mind can generate an infinite range of behavior by the muscles.

-Steven Pinker, The Blank Slate pages 36-37