A junior high school teacher was telling her class about evolution and how the way everything in the world was formed proved that God doesn't exist. She said, "Look out the window.  You can't see God, can you?"  The kids shook their heads.  "Look around you in this room.  You can't see God, can you?"  The kids shook their heads.  "Then our logical conclusion is that God doesn't exist, does He?" she asked at last, certain that she had won her audience over.

But one girl from the back of the classroom said, "Miss Smith, just because we can't see it doesn't mean it doesn't exist.  We could do brain surgery and investigate the parts of your brain and we could do a CAT scan and see the brain patterns in your head.  But we couldn't prove that you've had a single thought today. Does that mean that you haven't thought anything today? Just ‘cause you can't see it doesn't mean it doesn't exist." 

Seeing is believing, right?  But, "just ‘cause you can't see it doesn't mean it doesn't exist."