A Noun is a word used as the name of a person, place, or thing. An Adjective is a word used to add something to the meaning of a Noun. But, Adjectives are often used as Nouns.
1) As Plural Nouns denoting a class of persons; as,
The rich (=rich people) do not know how the poor (=poor people) live.
2) As Singular Nouns denoting some abstract quality; as,
The future (=futurity) is unknown to us.
3) Some Adjectives actually become Nouns, and are used in both numbers:-
a) Those derived from Proper Nouns; as, Italians, Canadians.
b) Some denoting persons; as, juniors, seniors, elders, criminals.
c) A few denoting things generally; as, solids, liquids, totals, secrets.
(Some adjectives are used as Nouns only in the plural; as, sweets, valuables.)
4) In certain phrases; as,
I do not want any more at present.
In short, we know nothing.