If I'm not mistaken, nouns (and nominals) are the only words that can inflect for grammatical number. E.g.: cat (Sg), cats (Pl); writing (Sg), writings (Pl).
"This" and "that" as singular demonstrative determiners have their plural counterparts, "these" and "those".
Are "these" and "those" actually inflected from "this" and "that" or are the two pairs different lexemes altogether?
If they were inflected, it would mean that this sort of inflection (or declension) is not exclusive to the part of speech 'noun'. If they were different lexemes, it would mean that the words have no relation in respect to grammatical number and inflection.
I am aware that there exist the demonstrative pronouns as well. The same question goes to this set of words too (pronouns and pronominals).