7e. Once that [that Bill had left] was clear, we gave up.As a linguist, I'm going to tell you that your naïve intuition that this sentence is ungrammatical is just because you're not smart enough to process the grammatical rules that you know subconsciously -- rules that are in fact mostly encoded in your DNA. What?
The sentence is odd for most native speakers: it is not acceptable. However, this sentence is formed according to the same principle that we posited to account for the formation of (7b)-(7d), i.e., that one sentence may become part of another sentence. Hence (7e) would be grammatical, though it is not acceptable.
Faced with intuitions such as that for (7e) the linguist might decide to modify the grammar he has formulated in such a way that sentence (7e) is considered to be ungrammatical. He may also decide, however, that (7e) is grammatical, and that the unacceptability of the sentence is due to independent reasons. For instance, (7e) may be argued to be unacceptable because the sentence is hard to process. In the latter case the unacceptability is not strictly due to linguistic factors but is due to the more general mechanisms used for processing information.
The native speaker who judges a sentence cannot decide whether it is grammatical. He only has intuitions about acceptability. It is for the linguist to determine whether the unacceptability of a sentence is due to grammatical principles or whether it may be due to other factors. It is the linguist's task to determine what makes (7e) unacceptable.
Seriously: I'd posit that, if your theory about a language doesn't account for what actual native speakers count as a valid sentence, then your theory is wrong! Is Haegeman representing the general Chomskyan position correctly here?
Our goal as scientists is to account for what happens observably. In what way does a proposed grammar of a language count as a falsifiable scientific theory if you can just say "in reality, that sentence is grammatical -- there were just processing difficulties"?