Poverty Of The Stimulus

Why did Chomsky use the ability to account for language learning as a defining characteristic of explanatory adequacy?  It was because Chomsky realized that language learning faced a key problem known as the poverty of the stimulus.  The poverty of the stimulus argument is the claim that primary linguistic data (i.e. the linguistic utterances heard by a child) do not contain enough information to uniquely specify the grammar used to produce them.  “It seems that a child must have the ability to ‘invent’ a generative grammar that defines well-formedness and assigns interpretations to sentences even though the primary linguistic data that he uses as a basis for this act of theory construction may, from the point of view of the theory he constructs, be deficient in various respects” (Chomsky, 1965, p. 201).  The poverty of the stimulus is responsible for formal proofs that text learning of a language is not possible if the language is defined by a complex grammar (Gold, 1967; Pinker, 1979; Wexler & Culicover, 1980).  It is the equivalent in linguistics to what is known as the problem of underdetermination in vision (Marr, 1982).

References:

1. Chomsky, N. (1965). Aspects Of The Theory Of Syntax. Cambridge, MA: MIT Press.
2. Gold, E. M. (1967). Language identification in the limit.  Information and Control, 10, 447-474.
3. Marr, D. (1982). Vision. San Francisco, Ca.: W.H. Freeman.
4. Pinker, S. (1979). Formal models of language learning. Cognition, 7, 217-283.
5. Wexler, K., & Culicover, P. W. (1980). Formal Principles Of Language Acquisition. Cambridge, MA: MIT Press.

(Added March 2011)