Studying Rene Descartes?
Well... if you think about when you are in your dream world, it's often hard to distinguish whether you're dreaming or experiencing reality. Our senses can often be deceiving. I know that, I myself, have experienced dreams where I felt real physical pain or heat from fire, etc. Our brains believe these things to actually occur, and so we feel or sense them to occur.
A lot of people also like to say that what we believe to be reality, isn't truly reality, and that what we experience as a 3d world can be altered if we truly put mind over matter. One could walk on water... I don't know about all that. But this is something a lot of Buddhist monks play around with.