Thank you for your comment. I think there is less of an epistemological problem for Descartes. I read his argument remembering that he is giving a version of Augustine's argument. Augustine phrased it better, saying "I could be deceived about my existence, but if I am deceived, nevertheless I am, because only if I exist could I be deceived." Being able to ask the question "do I exist" requires an existing being. Because Descartes generalized the argument, he left out the key to Augustine's argument and weakening his own.