matter vs mental object

matter and mental object both are nouns.

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

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

matter: some situation or event that is thought about
mental object: the sum or range of what has been perceived, discovered, or learned

Other hypernyms of matter include cognitive content, content.

matter (noun)	mental object (noun)
that which has mass and occupies space	the sum or range of what has been perceived, discovered, or learned
(used with negation) having consequence
some situation or event that is thought about
a vaguely specified concern
a problem
written works (especially in books or magazines)

matter (verb)	mental object (verb)
have weight; have import, carry weight