noumenon vs mental object

noumenon and mental object both are nouns.

Word	Noun	Adjective	Verb	Adverb
noumenon	Yes	No	No	No
mental object	Yes	No	No	No

As nouns, mental object is a hypernym of noumenon; that is, mental object is a word with a broader meaning than noumenon:

noumenon: the intellectual conception of a thing as it is in itself, not as it is known through perception
mental object: the sum or range of what has been perceived, discovered, or learned

Other hypernyms of noumenon include cognitive content, content.

noumenon (noun)	mental object (noun)
the intellectual conception of a thing as it is in itself, not as it is known through perception	the sum or range of what has been perceived, discovered, or learned