Quote Originally Posted by Frost.
Simple question with many possibilities,

"How do you know the world exists behind you?"

When you turn around you can no longer see anything outside of your front and peripheral vision. How do you know that the world actually exists when you turn around? How do you know that the world doesn't simply exists where you can see it?

A mirror, a camera, a friend telling you, etc. all have to exists in front of you for you to see what they are showing. As in, a mirror could show you what you think should be there however it really does not exists at that moment.

Where this is going is the thought that existance is simply a personal thing. The universe was created for you to enjoy it. Whatever you cannot see presently simply does not exist.

So, the question stands, how can 'you' prove that the world exists behind you?
Easy. Use a camera and take a picture of me straight on from above. Since while looking at the developed picture, you know that what you were looking at with your own eyes in front of you was real, the picture of you from the top (while you were looking forward) proves there's a whole world around you.