"It is well known by researchers that what we think we see can be strongly influenced by images and ideas we have been exposed to previously as well as our own thoughts and imagination. This is probably explains why it's the people who already believe in ghosts or UFOs who keep seeing ghosts and UFOs, and why so few nonbelievers do. Seeing things that are not there can happen to anyone because the brain constructs and interprets the visual reality that is around it."
Guy P. Harrison
Think: Why You Should Question Everything