For the nothing it seems to be worth, I agree that common experience is the only valid source of any commonality of knowledge--other than someone choosing to accept as true something one did not experience, such as accepting the word of a friend or findings of science or the spiritual experience someone purports to have had. No one can be forced to accept (genuinely) any proposed truth.

At the same time, though, our material circumstances as human beings, which include living together in groups, requires commonality of action--in ongoing governance and in response to material threats to the group. So the existential question seems to be how to achieve necessary commonality of action in the face of the ultimately radically individualistic nature of true knowledge.