On acceptability
A lot of research in linguistics relies on the notion of acceptability. During my work, I sometimes need to ask my non-linguist friends whether they think some sentence is acceptable or not. And most of the time they reply with another question: “Acceptable with respect to what?”. I repeatedly found myself unable to immediately give a satisfying answer. So I decided to write about it.
Oddities
Consider the following sentences:
(1) He boughts a new book
(2) He bought a new book
(1) no doubt sounds odd for any native English speaker. But why is it odd? The most common opinion seems to be “because no one speaks this way.” This is certainly a fact, and with respect to this fact, we can say that the sentence is unacceptable. By this logic, any Russian or Japanese sentence is equally unacceptable for a speaker of English and vice versa! There is nothing new or exciting about this kind of knowledge. After all, humans speak different languages, right? So what's all fuss about (un)acceptability?
However, it is not hard to notice a certain pattern here. In English, verbs in past tense do not agree in person with subjects. Once we put things this way, unacceptability of some sentences starts to seem to be related less to speakers and more to verbs, subjects and tenses. Linguists worked hard to find an enormous number of such “oddities” that can be explained in terms of categories and/or their positions in sentences.
One important thing here is contrast. Just like in any other experimental research, we need a “control group”. In syntactic research, this means providing the "oddity" along with the acceptable counterpart. So, in the case of (1), only if it is true that just omitting the -s makes the sentence (2) acceptable, we can say that (wrong) agreement is the real cause of unacceptability here.
One could object that it is only possible to give an explanation in terms of verbs, subjects and tense if one knows what verbs, subjects and tense are. Nothing in the sentence tells us that “bought” is a verb or that “he” is a subject. People just happened to be able to categorize them as such. Absent this knowledge, the no-one-speaks-this-way approach is perfectly fine.
This is not wrong. However, modern linguistics achieved much more than categorizing words into verbs or nouns -- and I think it tells us something. Of course, our current knowledge also essentially depends on our ability to notice more patterns and even patterns of patterns. But so does every other scientific knowledge! No one today asks why bother classifying myriads of fundamental particles in physics.
Relevance of acceptability judgements
But why focus so much on what no one ever says? The reason behind this focus is the ability of humans to have intuitions about language. In just a few years, children reach adult-like proficiency in their native language. In other words, the child comes to have adult-like intuitions about what is odd and what is not. How much of it is learned by simply listening and how much is already “there”, in his brain -- has been a central question in generative grammar for many years. We still do not have good answers. But it is tempting to think that exactly because children come “equipped” with something, they are able to reach adult-like proficiency so fast.
I hope this is enough to understand why we focus on these slight contrasts between acceptable and unacceptable sentences. If we can pinpoint the exact cause of the “oddity” then it might be related to the ability of a human to have correct intuitions. That's why it does not make much sense to ask why e.g. “Ghur hjhi erugheu” or some Japanese sentence is unacceptable in English or Russian. There is just too much “wrong” with it, and it is not very useful for discovering anything particular about our intuitions.
An accident
This one is my personal doubt.
What if all this is accidental? Suppose that tomorrow, all English-speaking population agrees that “He boughts a new book” is now acceptable. Their children will grow up hearing this sentence and, as a result, some future linguist will have to explain why “He bought a new book” is unacceptable in future-English. After all, past tense subject-verb agreement in person was totally a thing in Middle English (500+ years ago).
Now, we do not have any historical records of people doing that kind of sudden language-change meetings. The change likely happened at a much smaller scale and was gradual. Still, the principle is the same -- even a slip of the tongue might in theory start the chain reaction.
Today, some syntactic theories like phase theory partially rely on agreement patterns. But how wise is it to base our theories on something that is subject to change? One part of me feels that it is a very bad idea. The other part of me does not agree. Come to think of it, it is observations that would change, but not the way we would explain them anyway. For example, even if the acceptable/unacceptable pairs like the one above become reversed, we would still think that this is because of wrong agreement. This is definitely something I would like to think about more and maybe write another post about it.
Further reading
There are quite a few articles on the awesome Faculty of Language blog that tackle the problem of acceptability: