Here some theortical answers, Life is not a dream because the emotions are to real and the fact you can dream. The extreme levels of emotion would wake you up, just like when you exprince a.....errr.... erotic dream and you can dream within a dream, I mean have you ever?
As for the brain jar thing the whole would be why? I mean what could be gained by sticking your brain in a jar and giving you the life you have that couldn't be learnt by simple obreavtion of a whole human.
As for being certain of anything, what is certainy but the blief that something or some event, eixsts or happened/s respectly. Yet without knowing eveything we can never truly be sure.
So in conclusion we exist but can never be sure of anything, apart from our existance. (I think)
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The angel's MIA.