When a tree falls in a forest does it make a sound? No. Sound is the brain's perception of air molecules vibrating across a distance and for a duration. Yes, the tree still moves all those air molecules when it falls, but it makes no sound because there is nobody around to translate those vibrating molecules into sound. But then again I believe it is YES, because a fact (action/sound/whatever) is still there (or has been there), even if it cannot be "proved". The fact in itself doesn't need to be proved, to exist.