The realization that reality, as I perceive it, is a construct of my brain interpreting outside stimuli. My brain not only is a powerful stimuli interpreting machine, but also quite frequently produces its own stimuli, and as such could very well just be producing its own stimuli and interpreting it as reality. In other words, what I consider reality might just be a production of my mind and not be real. Life could literally be a dream, and I stay up late at night and wonder what this means far too often.