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Chomsky's Hierarchy of Syntactic Forms


In September 1956 American linguist, philosopher, cognitive scientist, and activist Noam Chomsky published "Three Models for the Description of Language" in IRE Transactions on Information Theory IT-2, 113-24. In the paper Chomsky introduced two key concepts, the first being “Chomsky’s hierarchy” of syntactic forms, which was widely applied in the construction of artificial computer languages.

“The Chomsky hierarchy places regular (or linear) languages as a subset of the context-free languages, which in turn are embedded within the set of context-sensitive languages also finally residing in the set of unrestricted or recursively enumerable languages. By defining syntax as the set of rules that define the spatial relationships between the symbols of a language, various levels of language can be also described as one-dimensional (regular or linear), two-dimensional (context-free), three-dimensional (context sensitive) and multi-dimensional (unrestricted) relationships. From these beginnings, Chomsky might well be described as the ‘father of formal languages’ ” (Lee, Computer Pioneers [1995] 164). 

The second concept Chomsky presented here was his transformational-generative grammar theory, which attempted to define rules that can generate the infinite number of grammatical (well-formed) sentences possible in a language, and seeks to identify rules (transformations) that govern relations between parts of a sentence, on the assumption that beneath such aspects as word order a fundamental deep structure exists. As Chomsky expressed it in his abstract of the present paper,

"We investigate several conceptions of linguistic structure to determine whether or not they can provide simple and “revealing” grammars that generate all of the sentences of English and only these. We find that no finite-state Markov process [a random process whose future probabilities are determined by its most recent values] that produces symbols with transition from state to state can serve as an English grammar. We formalize the notion of “phrase structure” and show that this gives us a method for describing language which is essentially more powerful. We study the properties of a set of grammatical transformations, showing that the grammar of English is materially simplified if phrase-structure is limited to a kernel of simple sentences from which all other sentences are constructed by repeated transformation, and that this view of linguistic structure gives a certain insight into the use and understanding of language" (p. 113).

Minsky, "A Selected Descriptor-Indexed Bibliography to the Literature on Artificial Intelligence" in Feigenbaum & Feldman eds., Computers and Thought (1963) 453-523, no. 484. Hook & Norman, Origins of Cyberspace (2002) no. 531.

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