When my son was much younger, he asked, "What if life really is a dream, and we are all sleeping, and when we die, we wake up?" (Yeah, and how do you answer that to a 6 or 7 yr old?!) I somehow doubt we are asleep -- yet often, in a dream, everything seems as real as waking time.
Then add to that the existence of false memories (our brains just hate those empty places, and love to fill in the gaps), the fact that we don't all experience pain the same (same stimulus to two different people would be rated differently) -- and who knows what else... how much can we trust what we perceive?