Imagine if you will that I have in my possession a box which I have not opened. Now imagine there is a ball in this box. This ball may be any color but I have claimed that the ball is red. Now since I have not seen the ball you may well wonder how I know this and you may well ask me "how do you know the ball is red?"

Let us further imagine that rather than opening the box and proving what color it is once and for all I reply "You can't prove that it isn't red".

Now you can't actually prove that the ball is not red. Should you simply accept my word?