The problem is that proof pertains only to rational methods. We can convince ourselves of the certainty of our subjective experience, but certainty isn't the same as proof. We've been certain about many things that turned out to be wrong. That was the shock of the Scientific Revolution. We were certain that the Sun orbits the Earth. Then scientists proved the facts were otherwise. Proof happens through objectification, whereas mystics deal in subjective certainties and the quality of life.