More specifically, people claim the existence of consciousness. It can't be held as an absolute irrefutable by investigation. To do otherwise would be violating several basic concepts of epistemology, science and thinking.
Surely experiencing that yourself adds more weight to others' claims?
The OP is suggesting that experience isn't first-hand knowledge the way it appears to be. Certainly he agrees that we appear to have first-hand knowledge, but this appearance is only a cosmetic illusion, a trick that lends some adaptive advantage. Something like: http://journal.frontiersin.org/article/10.3389/fpsyg.2015.00...