plural vs first principles

plural and first principles both are nouns.

Word	Noun	Adjective	Verb	Adverb
plural	Yes	Yes	No	No
first principles	Yes	No	No	No

plural (noun)	first principles (noun)
the form of a word that is used to denote more than one	the elementary stages of any subject (usually plural)

plural (adjective)	first principles (adjective)
grammatical number category referring to two or more items or units
composed of more than one member, set, or kind