2.1.7 Rules
The last thing we will mention in this section concerns the kinds of grammatical rules that could be responsible for producing structures such as in (34). Recall from the start of this section we introduced a formal rule which stated that sentences can be made up of words and other sentences:
(35) | sentence → word*, sentence* |
The rule states that a sentence is made up of some words and some sentences. Although this rule is not particularly accurate, we can see that this kind of rule is ideal for describing the kinds of structures we have been discussing, as they state what the immediate constituents of a structure are: in other words, this rule describes mother–daughter relationships.
From the structure in (34) it is possible to formulate the following rules:
(36) | S → DP VP | |
VP → V DP PP | ||
PP → P DP | ||
DP → D N |
Such rules are known as rewrite rules as they describe how to draw a tree by ‘re-writing’ the symbol on the left of the arrow for the symbols on the right. Thus, if we start with the S at the top of the tree diagram we can rewrite this as a DP and a VP. The VP can be re-written as a verb, a DP and a PP and the PP as a preposition and a DP. The DPs can then be re-written as determiners followed by nouns.
Although the system of rules in (36) is capable of describing the structures of a good number of English sentences, it is clear that we would need many more rules to attempt to describe the structures of all English sentences. For example, not every DP is made up of a determiner followed by a noun. Some may contain just a determiner, such as this for example, or a determiner, an adjective and a noun, such as a rusty kettle. A DP may indeed contain, amongst other things another sentence, such as the diagnosis that she had flu. It is clear that we would need many rewrite rules to capture all the possibilities for English DPs.
This fact does not invalidate this kind of rule for linguistic descriptive purposes. As long as there is only a finite number of rules, a legitimate grammar could be formulated even with a very large number of them. However, if human grammars are constructed of a large number of rules the question is raised of how children could ever learn their grammatical systems. This consideration has lead some linguists to assume that what is needed is a far more restricted set of rules. We will introduce the theory of phrase structure that follows this line of thought in the next chapter.