Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
Chomsky thought that two levels of language were needed: an underlying “deep structure”, which accounts for the fundamental syntactic relationships among language components, and a “surface structure”, which accounts for the sentences that are actually uttered. The latter gets generated by transformations of elements in the deep structure. For example, "I wrote this book" and "This book was written by me" use the same constituents ("I", "to write", "book") and such constituents are in the same relationship: but one is an active form and the other is a passive form. One gets transformed into the other. Their deep structure is the same, even if their surface structures are different. Many different sentences may exhibit the same deep structure.
The phrase structure produces the "deep structure" of a sentence. That needs to be supplemented by a transformational component and a morpho-phonemic component, which together transform the deep structure into the surface structure of the sentence (e.g. active or passive form).
Technically, the deep structure of a sentence is a tree (the “phrase marker”), that contains all the words that will appear in its surface structure. Understanding language, basically, consists in transforming surface structures into deep structures.
In Chomsky's "standard theory" a grammar is made of a syntactic component (phrase structure rules, lexicon and transformational component), a semantic component (that assigns a meaning to the sentence) and a phonologic component (which transforms it into sounds).
In the end, every sentence of the language is represented by a quadruple structure: the D-structure (the one generated by phrase-structure rules), the S-structure (obtained from the D-structure by applying transformational rules), the P-structure (a phonetic structure) and a “logical form”. The logical form of a sentence is the semantic component of its representation, usually in the guise of a translation into first-order predicate logic of the “meaning” of the sentence. These four structures define everything there is to know about the sentence: which grammar rules it satisfies, which transformational rules yield its external aspect, which rules yield the sounds actually uttered by the speaker, and finally the meaning of what is said.
Chomsky's computational approach had its flaws. To start with, each Chomsky grammar is equivalent to a Turing machine. Because of Godel's theorem, the processing of a Turing machine may never come to an end. Therefore, a grammar may never find the meaning of a valid sentence, but we have no evidence that our brain may never find the meaning of a valid sentence in our language. Therefore, some conclude that Chomsky's grammars are not what our brain uses. Also, Chomsky had to explain how we can learn the grammar of our own language: if the grammar is computational in nature, as Chomsky thought, then it can be proved mathematically that no amount of correct examples of sentences are enough to learn a language. It is mathematically impossible for a child to have learned the language she speaks!
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