Every human being that learned to speak learned this on the basis of a notion like fact.

The problem is not with the intuitive notion but with ascertaining whether or not statements represent facts, and what manner of facts, and how to ascertain this is so or not.

And indeed: Often one should know that the fact of a certain matter is that one does not have sufficient evidence to confidently assert what the facts are - and such knowledge that one has no knowledge about something, or not enough knowledge to pronounce confidently, is often the firmest knowledge one has.