In another (non-debate) thread, it was asked what 'real' is. A response to that question was the Google dictionary definition, "actually existing as a thing or occurring in fact; not imagined or supposed."
How can one be certain something is real given that definition? I'm fairly certain nearly everyone has had dreams that, while dreaming, they thought were real until they awoke.
What one perceives is merely a model resulting from sense organs that create electrical signals as interpreted by the brain. How can one trust that these are, indeed, real? How do you know you won't wake up from this reality into a 'real' one?