Editing Transformational grammar (section)

===Theory evaluation===
In the 1960s, Chomsky introduced two central ideas relevant to the construction and evaluation of grammatical theories.

==== Competence versus performance ====
One was the distinction between ''[[Linguistic competence|competence]]'' and ''[[Linguistic performance|performance]]''. Chomsky noted that when people speak in the real world, they often make linguistic errors, such as starting a sentence and then abandoning it midway through. He argued that such errors in linguistic ''performance'' are irrelevant to the study of linguistic ''competence'', the knowledge that allows people to construct and understand grammatical sentences. Consequently, the linguist can study an idealised version of language, which greatly simplifies linguistic analysis.

==== Descriptive versus explanatory adequacy ====
The other idea related directly to evaluation of theories of grammar. Chomsky distinguished between grammars that achieve ''descriptive adequacy'' and those that go further and achieve ''explanatory adequacy''. A descriptively adequate grammar for a particular language defines the (infinite) set of grammatical sentences in that language; that is, it describes the language in its entirety. A grammar that achieves explanatory adequacy has the additional property that it gives insight into the mind's underlying linguistic structures. In other words, it does not merely describe the grammar of a language, but makes predictions about how linguistic knowledge is mentally represented. For Chomsky, such mental representations are largely innate and so if a grammatical theory has explanatory adequacy, it must be able to explain different languages' grammatical nuances as relatively minor variations in the universal pattern of human language.

Chomsky argued that even though linguists were still a long way from constructing descriptively adequate grammars, progress in descriptive adequacy would come only if linguists held explanatory adequacy as their goal: real insight into individual languages' structure can be gained only by comparative study of a wide range of languages, on the assumption that they are all cut from the same cloth.{{citation needed|date=November 2013}}