I attended a training event on Tuesday, and the other people there were from various humanities and social science disciplines, none of them linguistics. We each had to present a talk on our research, and then there were questions afterwards. One woman, doing a fine art creative practice PhD (in ceramics), asked me this:
Is it important that everything is put into one category or another?Typical creative type, you might think, not wanting things to be 'labelled'. She meant the labels like 'noun', 'verb', 'question particle' and so on. And it's a perfectly reasonable question to ask. Why do things have to be categorised? Without the categories, there's no problem to have to be solved.
Well, it's not just linguists doing it for the sake of it, because we like things to be rigid and ordered and follow the rules (though some of us do). These categories, although the labels are artificial, are natural classes. Consider the birds (as Brian didn't quite say).
A goldfinch is a natural thing, I think we can all agree. It occurs in nature without human intervention (all right, it's been captured, bred and sometimes escaped, and introduced to various bits of the world, but it wasn't created by humans).
This goldfinch is called Harold. That category (Harold) consists of just this one individual. Goldfinches generally, though, have a specific scientific classification: Carduelis carduelis. That's its species. Then we can identify the category of passerines, the larger category of birds, and the even larger category of animals. There are other categories in between too, each of which Harold is a member of.
The point is, although the names for these categories were chosen and imposed by humans, the categories themselves were not. Humans observed similarities between organisms, and those that share characteristics form a natural class (for example, birds, which have feathers and wings, or living things, which have the characteristics of MRS NERG - movement, reproduction, sensitivity, nutrition, excretion, respiration, growth). Everything in nature is categorisable, and not because scientists have imposed those categories, but because they really are a category.
In the study of language, we do the same. We say that all nouns are nouns because they behave like nouns, they share all the characteristics of nouns, and they are different from things that are not nouns. We called them nouns, but the category was there before we came along and noticed it and named it.
I didn't give the questioner an answer quite as full as this because I couldn't think on the spot. But I hope I got across to her the message that we're not just making all this up - language is a real, natural object. We're just describing it and trying to explain how it is the way it is.