[…] the empirical considerations that enter into the choice of a theory of PF and LF fall into two categories: grammar-internal and grammar external. In the first category, we ask how particular assumptions about PF and LF relate to the rules and principles of grammar; in the second, we ask how such assumptions bear on the problem of determining physical form, perceptual interpretation, truth conditions, and other properties of utterances, through interaction of PF, LF and other cognitive systems.
- Chomsky (1993), Pag. 4