5.4.5 Recursion and the infinitude of language

　 At the beginning of this chapter, a point is made that a TG grammar must account for all and only grammatical sentences. Phrase structure rules, as an important component of such a grammar, must be adequately formulated so as to generate all and only grammatical sentences. The four rules generalized in 5.4.3 are tentative. We can readily give some sentences which are grammatical but not accounted for by our tentative rules:

　　(34) I don't know the poor boy works.
　　(35) He lives in the old house by the road.
　　(36) I saw the poor boy who delivers local papers.

English speakers agree that these are grammatical. Then the rules must be revised in order to account for sentences like these. Based on sentences like (35) and (36), the NP rule should be revised as:

　　Now, our revised phrase structure rules show that S contains NP and VP and that S may be a constituent of NP and VP. Rules (ii) and (iv) show that NP and PP can be mutually inclusive. If phrasal categories appear on both sides of the arrow in phrase structure rules, the rules are recursive. Recursive rules can be applied again and again, and the phrase structure trees can grow endlessly. Look at the following tree diagram:

Within the S there is an embedded S, the poor boy works. This sentence can be embedded in another sentence You know I don't know the poor boy works. The following tree shows the PP-NP-PP-NP recursion:

We can easily extend the sentence like this: The boy lives in the old house by the road along the river across the town. Recursion is found in other phrasal categories, but we will not continue the discussion. What can be seen is that phrase structure rules account for the creativity and infinitude of language, which earlier approaches to syntax fail to reveal in linguistic representation of sentence structure.