ideas are also "nouns", i've been told. ideas are also things, some things are ideas, some people are places, some things are places, some places are ideas. clearly the common definitions for these words have a large amount of overlap semantically speaking.
linguistically it is defined as a part of speech that is the main component of the object of a preposition or verb, or the subject of a clause.
i suppose you could say it is redundant, yes. different language families categorize their morphemes differently, some for example have no adjectives at all. certain native american languages use verbs that function in their place. the distinction between adverb and adjective is also not universal.
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