Levels of adequacy

In his seminal work Aspects of the Theory of Syntax (1965), Noam Chomsky introduces a hierarchy of Levels of Adequacy for evaluating grammars (theories of specific languages) and metagrammars (theories of grammars).

These levels constitute a taxonomy of theories (a grammar of a natural language being an example of such a theory) according to potency. This taxonomy might be extended to scientific theories in general, and from there even stretched into the realm of the aesthetics of art. This present article's use of the phrase as a terminus technicus should not be confused with its everyday language uses.

The "potency" criterion alluded to in the preceding section is somewhat ill-defined, but may include "exhaustiveness", "effectiveness', and an affective component as well. (Arguably, the taxonomy is also motivated by considerations of "elegance". This should not be confused with the application of the taxonomy in the field of aesthetics). As a metatheory, or "theory of theories", it becomes a concept of epistemology in the philosophy of science, rather than a mere tool or methodology of scientific linguistics. As Chomsky put it in an earlier work:

"The theory of linguistic structure must be distinguished clearly from a manual of helpful procedures for the discovery of grammars."

"...the grammar gives a correct account of the linguistic intuition of the native speaker, and specifies the observed data (in particular) in terms of significant generalizations that express underlying regularities in the language."

"A linguistic theory that aims for explanatory adequacy is concerned with the internal structure of the device [i.e. grammar]; that is, it aims to provide a principled basis, independent of any particular language, for the selection of the descriptively adequate grammar of each language."

Theories which do not achieve the third level of adequacy are said to "account for the observations", rather than to "explain the observations."

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