languages, where complementizers are required. So a child raised in an English-speaking home might hear the sentences of (7a–c) pronounced with or without the complementizer that. These experiences would show them that that is optional so they will learn a complementizer-deletion rule like (7d).
(7) a. Peter said [CPthat/0 [IPKay left]].
b. Kay doesn’t believe [CPthat/0 Ray is smart].
c. It was obvious [CPthat/0 [IPKay left]].
d. That →0
So far, we can see that children learn different things about their native languages, depending on the input they are exposed to. However, children also know things about their language that they could never have learned in this way. In fact,thatis not always optional; it is required in the con- texts of (8), where the asterisk next to the zero means that deletion is not possible.
(8) a. Peter said yesterday in Chicago [that/*0 Kay had left].
b. Fay believes, but [IPKay doesn’t [VP[V e [CPthat/*0 Ray is smart]]]].
c. [that/*0 Kay left] was obvious to all of us.
Something in the English grammar – and therefore in the English speaker’s brain – requires the complementizer that in these kinds of sentences. But no adult or child ever says Kay left was obvious to all of us, so no one can point out that this is an ungrammatical sentence in English.
This is called negative evidence, information about what does notoccur in a language. They hear sentences like (8) only with complementizers, but they can’t possibly learn from what they hear that the complemen- tizer is notoptional in just the contexts in (8). Children acquiring English
“know” that the cases in (8) are impossible, but since they don’t have evidence to learn this from, this knowledge must be coming from some- where else. A straightforward answer is that the mental organ for language – which many linguists call Universal Grammar or UG – must be playing some role. This conclusion matches the scientist’s conclusion that the robot must have been programmed with basketball rules, because it hadn’t seen enough evidence to learn them by watching basketball games.
The facts that we are presenting may seem too commonplace to require explanation. “Of course you can’t say ‘Kay left was obvious to all of us’,”
you might think, “Anybody can see that.” But in Isaac Newton’s time, everybody knew that apples fall down, and nobody thought it was any- thing that needed to be explained. Newton advanced the field of physics and made a place for himself in history by asking “Why?” Linguists are doing the same thing when we ask questions about that-deletion. The results we get are surprising in the same way that the discovery of gravity was, though they are not yet as widely known.
The problem of thatcomplementizer deletion is that thatcan readily be deleted in English in some contexts, but not in others. To see why, let’s
consider (7a) in detail. Figure 3.14 is the verb phrase part of (7a). We’re assuming that the abstract inflection PASThas already become part of the verb leave giving the past-tense form left. The same thing has happened with said. If you look carefully at Figure 3.14, you will see that [CP(that) [IP Kay left]] is the complement of the verb phrase, and also directly adjacent to the head of that verb phrase, said.
The complementizer that can be omitted if its clause is the complement of an overt, adjacent word. An overtelement is pronounced, like thatin Peter said that Kay left, as opposed to Peter said Kay left, in which the com- plementizer is structurally “there,” but not pronounced. The same is true of the other examples in (7), where thatdeletion is possible. In (8a), on the other hand, the clause is still the complement of say. But the complement is not adjacent to its head, say. Chicago is not the head of the verb phrase and the CP containing Kay leftis not the complement of Chicago. In (8b) the CP containing Kay leftis the complement of the elided but understood verb believe, which is not overt(the eagain means empty), and in (8c), the clause is the complement of nothing. Children cannot learn all this, because they would have to be able to use data about what does not occur (negative data).
If we just look at this narrow range of facts, it seems that the following principle is part of Universal Grammar.
(9) Thatcomplementizers, if they are deletable, can be deleted if they are adjacent to an overt head and they are in that head’s complement.
With this principle, we solve this particular poverty-of-stimulus problem.
Children learning English learn that the that complementizer can be
FIGURE3.14
That Key leftas part of the complement of said and adjacent to said
deleted because they hear some kinds of sentences in which that may appear, but doesn’t have to. In other types of sentences, like the ones in (8), they always hear that, never examples of its deletion. But the condi- tions under which that cannot be deleted depend on some very subtle properties of grammar, involving notions like a clause being a comple- ment of some audible word, and nothing like this is ever part of children’s experience. Since children are sensitive to it anyway, we are forced to con- clude that they “know” this principle already.