Intermediate Cases

An epigram:
"We may assume for this discussion that certain sequences of phonemes are definitely sentences, and that certain other sequences are definitely non-sentences. In many intermediate cases we shall be prepared to let the grammar itself decide, when the grammar is set up in the simplest way so that it includes the clear sentences and excludes the clear non-sentences." -Chomsky 1957, p. 14.

The idea:
The grammaticality of some sentences can't be determined easily by humans. So we make our grammar without considering these sentences. Then, once we have our grammar, we can look at these sentences to determine their grammaticality.

Some thoughts:
So this seems initially reasonable; people do this all the time. For instance, let's say you see someone who you can't tell the gender of. There is a theory that says there are certain body parts that men have and others that women have. This theory was built up based on observations from certainly-males and certainly-females. So we go to this androgynous person and see what body parts they have to determine their gender. If we want to be even more scientific about it, we could do genetic testing, look at their chromosomes, and have our answer.

Of course, nothing in life is easy like this - even in genetic testing there are border cases. For instance, there's Klinefelter's Syndrome, where a person has two X and one Y chromosome. Categoricality just isn't there - there're always intermediate cases. And so it's almost certainly not right to say there are grammatical and ungrammatical sentences and nothing in between. But let's just suppose that these strict categories exist for now.

The bigger problem is that grammar isn't separate from us. There is no such thing as a "right" grammar of English. (To the dismay of many old fuddy-duddies.) There is no oracle that we can go to and ask "Is this sentence grammatical?" What we call the grammar of a language is really the consensus of the idiolects of its speakers. the grammar of a language is what we say, not what we think we say, nor what we think we should say, nor what a theoretical construct says we should say. A cornerstone of modern linguistics is that our grammars are descriptive, not prescriptive. A descriptive grammar predicts what we think about sentences; it can't tell us what to think. If there is an intermediate case, a good grammar must predict that.

Returning to the gender example, asking a grammar to determine the grammaticality of an intermediately-grammatical sentence is akin to asking a theory to determine whether a person suffering from Klinefelter's Syndrome is a man or woman, when the person isn't entirely in either category. Or like saying that physics should be based on waves and particles and thus should be able to settle once and for all which one light is. But, it's both. It's intermediate. And we have to build theories around that. Likewise, we as linguists have to make our theories fit the intermediacy of some sentences, rather than asking the theories to tell us which way those sentences go.

I think that's what's problematic with Chomsky's idea, which is sort of ingrained in us (me, at least): our theories can't tell the data what to do.