You've probably ever dreamed of this, at least once: You are either falling from a high building, getting shut at or endorsing any kind of (fatal) endings. Now, before that happend you've probably saw, in that dream/nightmare, some acurate details of the story and the events leading to the (fatal) end. While falling, feeling pain etc you were probably sweating, your heart rate was accelerating and had other physical reactions to the images projected in your mind while sleeping. You wouldn't sweat or be afraid if you were not convinced (at that moment) that it(your dream) was 'relaity'! Now: If you believe, at least during that dream, that what you're experiencing is reality and was fully believing it: Why wouldn't your current existence be a dream (of somebody else) too?