| What you live in all the time, regardless of whether you realize it or not.
Everything you see, hear, taste, feel, and smell is in your mind. You may think you see the monitor in front of you - but you don't. Your mind creates the image of it from signals from your nerve cells. There is, in fact, a miniature version of reality in your mind at all times, where the signals are processed by your brain and constructed.
Imagine living in a suit. That suit is opaque so you can't see anything, but the inside is covered in monitors that display what's on the other side. It has speakers that play the sounds it picks up with microphones. Sensors that detect touch and transfer it to your skin. And same with smell and taste - and it transfers everything back out of it in the same manner. If the suit works completely properly, then you could probably not even tell you had it on - but you'd still be one more layer away from reality, and there's always the chance that what it shows you isn't exactly what's happening. It's the same sort of thing - your body is a vehicle for your mind.
See also: the homunculus problem | 
