What is reality? How can we define it - fit it into a box - so that whatever experiments we throw at it, our definition always holds true?
I consciously observe the lucid dream world. It is real to me because the firing of neurons in my brain stem are interpreted as real sensory data by my brain. I could argue that lucid dreams constitute part of my reality.
But what if no-one else can perceive my dream reality?
Just how many realities are there anyway - yours, mine, his, hers?
As Einstein suggested, is every form of reality merely an illusion? Is nothing real?