Are parts of speech sharply defined categories? Can a single word be included in different parts of speech?

Question

I have been trying to figure out how words are categorized into parts of speech. The way I understand it, words can only be included into various parts of speech depending on their meaning and usage in a sentence. So It's not possible to figure out what category a word falls into just looking at the word rather you have to put them into a sentence. Words like act, laugh, attack etc. can be both verb and noun at the same time inspected in isolation.

But it turns out that even analysing a sentence cannot determine what category they fall into. See the example John Lawler gives in this related question: https://english.stackexchange.com/questions/388099/how-many-parts-of-speech-can-a-word-be-at-the-same-time

1. I was exhausted.
2. I was exhausted and the bed was soft; we suited each other well.
3. I was exhausted by the irritable conversation and left early.

"But that's not "in the same sentence". In the first sentence, there's just no way to know what the speaker intends about POS; it could be either one. And there's no way to know if one speaker might feel it was an adjective, but another speaker might think it was a participle. Or the same speaker might do both, to the same sentence, on different occasions."

Here the word exhausted is both a participal adjective and a verb at the same time. But can't you say that the adjective("exhausted") was formed from the verb exhaust and so the word here is "primarily" a verb which can act as an adjective. Wouldn't this allow us to put the word "exhaust" and "exhausted" into a primarily verb category? This is in some sense a word formation analysis to determine what the essence/category of a word is.

But one another sentence where this technique doesn't happen to work is:

"It is light."

You can see that "light" works here both as a noun(as in "bright light") and an adjective(as in: "That bag is light") but I can't seem to find a primary meaning to the word which I think would require a historical analysis of the word formation to categorise. I am not even sure if this a proper technique to categorise these words and it feelslike I am shoehorning words here into exclusive categories.

In short: Are parts of speech really sharply defined exclusive categories where a single word (when analysed within a sentence) falls into a single group or are they heterogeneous categories where words get shared within parts of speech categorizations?

Update: Adding some pointers if someone stumbles on this question in the future.

On further inquiry, I came to know that this problem is documented as Intersective Gradience: https://www.google.co.in/books/edition/The_Oxford_Dictionary_of_English_Grammar/9zVOCgAAQBAJ?hl=en&gbpv=1&pg=PA184&printsec=frontcover&bsq=Gradience

Also see Bas Aarts' Syntactic Gradience for a thorough take on the issue, By the end of his book there is a section on true hybridity which is approachable and could give you a clearer picture on the issue: https://www.google.co.in/books/edition/_/pN5zQgAACAAJ?hl=en&sa=X&ved=2ahUKEwizjpyDlcrtAhVSyzgGHc-FDvMQre8FMBF6BAgLEAM

A brief intro to Gradience: https://www.thoughtco.com/gradience-language-term-1690906

Answer 1

The fact that some sentences in English are ambiguous is not surprising. All languages have sentences that are ambiguous.

In English, this ambiguity is caused by the fact that English is a largely analytic language, with syntax and word order being more important than word endings. In simple and short sentences it may be possible to parse them in different ways, each of which may be correct.

So, as adjectives aren't particularly marked, and nor are nouns. And as both adjectives and nouns can be the complement in an "It is..." sentence, it is possible that a sentence like "It is light" is ambiguous and the word "light" could be an adjective, or it could be a noun.

Does anybody find this dialogue confusing?

Tell me about that bag of shopping that you are carrying.

It is light!

How about:

What do you think is the most important thing in making a room feel spacious?

It is light!

Of course in these (rather artificial) contexts there is no chance of misunderstanding. True ambiguity of grammar is very rare. True situations in which you can't parse a sentence in context hardly ever happen. This is as true in English as it is in every other natural language.

Every language has ambiguities. Every speaker of every language has no problem resolving the ambiguities in context.

English is no more a mess than any other language.

Answer 2

Here are two statements that I think do not get us lost in opinions on what is the “right” definition.

Participles in English are words formed from root verbs but are not themselves verbs. That is primarily a descriptive definition

Or we could define functionally participles as words that can be used as parts of verb phrases, as adjectives, or (if a present participle) as nouns. In some cases, particularly with respect to sentences where the verb is a form of “be” and the participle is a passive participle, it makes no difference to meaning whether the participle is classed as part of the verb or as an adjective complementing the subject.

Taxonomy is not an absolute; it is tool. It is best to think centers rather than boundaries.

EDIT IN RESPONSE TO COMMENT

In some languages, it may be true that each meaning of a specific word must represent the same part of speech, but that is just false in English. What determines part of a speech in English is meaning, not sound.

Look in any English dictionary, and you will find many words that have multiple meanings. "Light" for example has, according to Merriam Webster, six primary meanings, one of which is a noun, one of which is an adverb, two of which are adjectives, and two of which are verbs. (I have ignored the many secondary meanings: MW gives 16 of those just under noun.)

Consequently, it makes little or no sense to ask the question "what part of speech" is an English word when different meanings of that word represent different parts of speech. What makes sense is to ask what part of speech is a particular meaning, and, in fact that is exactly how English dictionaries proceed.

One could, I suppose, say that the meaning of a word most frequently used determines the primary part of speech for that word, but that seems to have little utility and is highly dependent on the speakers sampled. "Light" would then be primarily a noun for physicists and primarily an adjective for furniture movers although physicists would also use the word in its adjectival meaning and furniture movers would also use the word in its meaning as a noun.

Why English has many words with multiple meanings may indeed have to do with the substantial loss of inflections, total loss of grammatical gender, dual roots in Anglo-Saxon (a Germanic language) and the Old French dialect of Normandy (a Romance language), remnants of ablaut, and the Great Vowel Shift, but knowledge of such historical facts is of no help in understanding modern English: over 99% of English speakers have no knowledge of everything on that list, yet manage to speak English all the same.

Does the fact that so many English words have multiple meanings create a problem for people learning English? Undoubtedly. Does it mean that it is possible to utter ambiguous statements in English? It is quite possible, but usually context resolves the ambiguity.