mental object vs cognition

mental object and cognition both are nouns.

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

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

mental object: the sum or range of what has been perceived, discovered, or learned
cognition: the psychological result of perception and learning and reasoning

Other hypernyms of mental object include knowledge, noesis.

mental object (noun)	cognition (noun)
the sum or range of what has been perceived, discovered, or learned	the psychological result of perception and learning and reasoning