Are there a finite number of noun phrase rules for nlp?

Question

For example, a noun phrase might break up into these ways:

You should eat [noun phrase]
You should eat fish
You should eat the fish
You should eat the fresh fish

You should eat NP(N)
You should eat NP(Det, N)
You should eat NP(Det, Adj, N)

Is there a finite number of possibilities for the way a noun phrase might be constructed? Or is there an infinite number of possibilities?

I'm wondering because I'm starting to see papers that talk about machine learning techniques for noun-phrase detection, but if there are a finite amount of combinations, then it seems someone would have a complete list of rules or something out there.

What do you think?

Answer 1

A noun phrase is only what you (or the linguist you learn from) define it to be. Different understandings of a language could possibly lead to different definitions of what constitute a noun phrase.

Given that you have some precise idea of what constitute a noun phrase, you then have to express that idea so that you can communicate it to others. To communicate it precisely, you need to express it with some kind of formal (mathematical ?) structure or notation.

There are many ways to do that, a simple one being phrase structure grammars introduced by Chomsky half a century ago.

Such a grammar is constituted of rules describing how phrases (a noun phrase for example) are built by putting together (sub-)phrases and parts of speech (words).

So for example you could add to your lists of examples

You should eat NP(NP, Prep, NP)

for the sentence

You should eat fish from the blue sea

This is an example of a noun phrase composed with two other noun phrases "fish" and "the blue sea", and a preposition, according to a recursive rule. Examples can be more complex and involve different kinds of phrases.

Such recursive rules can allow you to make arbitrarily long noun phrases, and thus you can have an infinite number of noun phrases, though all defined with a finite number of rules (of ways to construct them): I added only one rule to your set.

Now, whether you have an infinite or a finite number of phrases of a given type depends on the way you understand your language and wish to describe it. In general, people do understand languages as having an infinite number of noun phrases, which can be characterized by a finite number of rules.

I insist on the fact that it all depends on the linguist's understanding because there is not "a complete list of rules or something out there".

Actually there are usually many such lists for a given language, corresponding to different understandings of the structure of the language, corresponding to different linguistic theories. It does not show too much for simple noun phrases, but the complete description of a language goes far beyond that.

Furthermore, there are also different ways of expressing the structure of the language, with rules or otherwise. The world is a complicated place.

Answer 2

Actually, it can be easily proven that there are an infinite number of possible noun phrases, AND also that that infinite set can be generated by a finite number of (recursive) rules.