Scenario: A magician is holding a prop behind a towel, so that the audience cannot see it.
Magician: There is nothing in my hand. If there were, you could see it.
Audience: All we can see is the towel, which is concealing whether there is anything in your hand.
Magician: I realize that the towel has concealing properties, but, for all you know, there is nothing in my hand.
That Guy: There is nothing in his hand. All we can see is the towel.
Audience: The towel prevents us from determining whether there is anything in his hand.
That Guy: Do you see anything in his hand? No. So we know there is nothing there.
Audience: That isn't how this works.
That Guy: I don't want to argue semantics.