CONCERNING THE BASE COMPONENT OF A TRANSFORMATIONAL GRAMMAR
The base component of a transformational grammar, which specifies what structures are available for the transformations to operate on, has hitherto been regarded by Chomsky and others as involving rewriting rules. In Chomsky's most recent treatment (Chomsky, 1965, henceforth Aspects) the base component consists of two subcomponents: a constituent structure sub component, consisting of rewriting rules and rules of another type which will be discussed below, and a lexicon. In earlier treatments by Chomsky (Chomsky, 1957, 1962) the lexicon is itself considered to consist of rewriting rules and the base component is regarded as a single system of rewriting rules rather than something consisting of two subcomponents. The term 'rewriting rule' is borrowed from the mathematical theory of rewriting sys tems. A rewriting system is a finite set of rules (p where (p and 1/ are strings of symbols, and a derivation in a rewriting system is a sequence x1, x2, xn of strings of symbols such that the first string (or 'line' x1 of the derivation consists of the single symbol S and each subsequent line can be obtained from the preceding one by substituting a string of symbols /t for a string 9, where (p is a rule of the rewriting system. A derivation is said to be terminated if its last line consists only of symbols from a preassigned set called terminal symbols (the other symbols which appear in the rules are called non terminal symbols) The last line of a terminated derivation is said to be generated by .the rewriting system, and the language generated by the rewriting system is the set of all strings generated by it. It should be emphasized that even though the base component of Chomsky's earlier versions of syntactic theory consists entirely of rewriting rules, it is not a rewriting system, since Chomsky imposes an ordering on the rules of the constituent structure component, whereas the term 'rewriting system' refers to an unordered system of rewriting rules (Chomsky, 1963) Thus, while in a rewriting system any derivation is permitted in which each