even if you are mistaken as to who 'you' are....you still are perceiving something (even though incorrect or inaccurate). Descartes did a thought experiment called 'critical doubt' where he discarded everything that he could not prove without a shadow of doubt. He used a couple motifs to do it....most famously...that his brain was floating in space in a vat hooked up to a machine that made him THINK he was on earth in his study (ie The Matrix). What is logically undeniable is that he THINKS he's in his study. *because he can't prove he actually IS in his study. And from this singular truth he can prove that he exists...whether his perception of reality is reliable or not.