public knowledge vs cognition

public knowledge and cognition both are nouns.

Word	Noun	Adjective	Verb	Adverb
public knowledge	Yes	No	No	No
cognition	Yes	No	No	No

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

public knowledge: knowledge that is available to anyone
cognition: the psychological result of perception and learning and reasoning

Other hypernyms of public knowledge include knowledge, noesis.

public knowledge (noun)	cognition (noun)
knowledge that is available to anyone	the psychological result of perception and learning and reasoning