first principles vs plural

first principles and plural both are nouns.

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

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

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