If some people do know, they are either unable or unwilling to prove that what they feel they know is in fact true. Experience alone is subjective, the brain generates all sorts of experiences that feel real, but are brain generated.
After all, it is the brain that constructs us and our experience of the world from sensory inputs and memory in every moment that we are awake, going about our daily activity.
Our experience of self and the world is virtual construct being generated by a brain, a virtual representation of the objects and events of the objective external world (which happen regardless of what we believe), and our place in it.
