How do you know if reality is reality?

... And then I woke up. I woke up? But I wasn't dreaming. No bed. No ... _gravity?_ The misty surroundings chocked me. The shock of being in this ... well it's not ghastly enough to be hell, and heaven - well this _isn't_ heaven. My whole life ... a dream? "If only Rosie was here she could explain everything" I murmured to myself. I've never known a 'Rosie' but my heart says she existed, exists. Then I realised it. The very thought frightened me. I have two lives. My dreams. My ... here, now, whatever you call this. My life started to come back to me. The one here. What is this?