Chapter 3 - Basic Concepts of Syntactic Theorysame as an adverb. For example, even when PPs are used adverbially, they still have
different distributions to AP:
(9) a we met [AP secretly] we met [PP in secret]
b we [AP secretly] met *we [PP in secret] met
As we see in (9), a PP modifier of a verb must follow it, while an AP modifier may
precede or follow it, even if the two modifiers have virtually the same interpretation.
Thus a phrase headed by a preposition has a different categorial status to one headed
by an adverb, supporting the X-bar claim that phrases have heads of the appropriate
kind.
Moreover, the X-bar rules in (1) rule out another possibility if we assume that these
are the only rules determining structure. While it might not make much sense to have a
phrase with a head of a different category, the idea of a phrase that simply lacks a head
is not so absurd. There is a traditional distinction made between endocentric and
exocentric language elements. An endocentric phrase gets its properties from an
element that it contains and hence this element can function by itself as the whole
phrase. For example:
(10) a I saw [three blind mice]
b I saw [mice]
An exocentric phrase on the other hand contains no element that can have the same
function as the whole phrase and so appears to have properties that are independent
from the elements it contains. A standard example is:
(11) a we saw him [in the park]
b we saw him [in]
c we saw him [the park]
The issue is rather complex. The traditional view mixes category and function in a
way that is perhaps not helpful. The point is, however, that the X-bar rules in (1) claim
that, categorially, all phrases are endocentric: in other words, all phrases have heads
which determine their categorial nature.
There is one grammatical construction that seems at first to stand outside the X-bar
system precisely in that it lacks a head: the clause. Certainly from a functional
perspective the clause contains no element that could replace the whole construction:
neither the subject nor the VP can function as clauses by themselves:
(12) a [Susan] [shot Sam]
b [Susan]
c [shot Sam]
The examples in (12) all have very different natures, even categorially.
It might be argued that sometimes VPs can act as clauses:
(13) get out!
However, such expressions have a special status and there is more to them than
appears at the surface. The sentence in (13) is an imperative construction in which
there appears to be no subject. However it is fairly clear that there is a definite subject