not. First, a grammatical sentence need not be included in a corpus. Secondly, it need not be meaningful. Finally, it does not have to be statistically probable. Chomsky shows all three points using a nonsensical sentence "Colorless green ideas sleep furiously." He writes that the sentence is instinctively "grammatical" to a native English speaker. But it is not included in any known corpus at the time and is neither meaningful nor statistically probable. Chomsky concludes that "grammar is autonomous and independent of meaning." He adds that "probabilistic models give no particular insight into some of the basic problems of syntactic structure." Carnap's influence British linguist Marcus Tomalin stated that a version of "Colorless green ideas sleep furiously" was suggested decades earlier by Rudolf Carnap. This German philosopher offered in 1934 the pseudo-sentence "Piroten karulieren elatisch". According to American linguist Reese Heitner, Carnap's sentence showed the autonomy of both syntactic and phonological structures. Grammar models and transformations In the third chapter titled "An Elementary Linguistic Theory", Chomsky tries to determine what sort of device or model gives an adequate account of a given set of "grammatical" sentences. Chomsky hypothesizes that this device has to be finite instead of infinite. He then considers finite state grammar, a communication theoretic model which treats language as a Markov process. Then in the fourth chapter titled "Phrase Structure", he discusses phrase structure grammar, a model based on immediate constituent analysis. In the fifth chapter titled "Limitations of Phrase Structure Description", he claims to show that both these models are inadequate for the purpose of linguistic description. As a solution, he introduces transformational generative grammar (TGG), "a more powerful model ... that might remedy these inadequacies." Chomsky's transformational grammar has three parts: phrase structure rules, transformational rules and morphophonemic rules. The phrase structure rules are used for expanding grammatical categories and for substitutions. These yield a string of morphemes. A transformational rule "operates on a given string ... with a given constituent structure and converts it into a new string with a new derived constituent structure." It "may rearrange strings or may add or delete morphemes." Transformational rules are of two kinds: obligatory or optional. Obligatory transformations applied on the "terminal strings" of the grammar produce the "kernel of the language". Kernel sentences are simple, active, declarative and affirmative sentences. To produce passive, negative, interrogative or complex sentences, one or more optional transformation rules must be