That's true, as Feynman put it, you are never absolutely sure of anything in the physical world, everything is reasonably correct by some probability and it's this kind of thinking that is hard to achieve for a normal person, but absolutely essential for a scientist.
That's why I am an agnostic, I am not entirely sure of the existence of God, but I am reasonably sure that he/she doesn't exist!
Well that depends entirely on which theory of gravity you believe in.
I personally like the whole anti-theory about gravity being produced by vacuums, because when you argue it against someone unknowlegable it makes pseudo-sense. However anyone with any clue of physics knows this is the easiest debunked theory in the world. I mean vacuum tube factories would have exploded in anti-gravity force.
