It's an interesting question, are we in a Matrix, or computer simulation, or the slightly older theory from 1641 where Rene Descartes imagined an omnipotent demon lying to him. Maybe I'm simplifying the question, but I reckon it boils down to how do we know what we know? Am I sat in front of a computer screen now, or do I just believe I am?

I'm not really a philosopher, at all, but the Oxford Companion to Philosophy says:

"Do you know that you are looking at a... book right now rather than, say, having your brain intricately stimulated by a mad scientist? The sceptic carefully describes this alternative so that no experiment can refute it. The conclusion that you really are looking at a book, however, explains the aggregate of your experiences better than the mad scientist hypothesis or any other competing views."

which I think makes sense to me. I've met one or two people who say they believe that they live in a Matrix/Alien computer simulation, but observing, they live as if they don't.