On a much deeper and hence much more rarely attainable level ( that of explanatory adequacy), grammar is justified to the extent that it is a 'principled' descriptively adequate system, in that the linguistic theory with which it is associated selects this grammar over others, given primary linguistic data with which all are compatible. In this sense, the grammar is justified on 'internal' grounds, on grounds of its relation to a linguistic theory that constitutes an explanatory hypothesis about the form of language as such.
- Chomsky (1969), a pag.27