knowledge base vs mental object

knowledge base and mental object both are nouns.

Word	Noun	Adjective	Verb	Adverb
knowledge base	Yes	No	No	No
mental object	Yes	No	No	No

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

knowledge base: the content of a particular field of knowledge
mental object: the sum or range of what has been perceived, discovered, or learned

Other hypernyms of knowledge base include cognitive content, content.

knowledge base (noun)	mental object (noun)
the content of a particular field of knowledge	the sum or range of what has been perceived, discovered, or learned