Existence: Exclusive to the mind

Descartes says that we think therefore we are. Because we are able to ponder the idea of our own existence or anything for that matter proves that we exist. If I were non existent then there would be no thought. Does this mean that the only thing that exists with any certainty is the mind, and outside of the mind there is only doubt? With no grasp on what, other than self, is real then why bother with rules, regulations, and expectations. If nothing is real for certain then there are no consequences for the things that one does because those things which were done may not have been done at all, or may have been done to non existent things... therefore it yields no consequences. For example: When Dr Layne experiences sleep paralysis she is ,in her head, screaming at her husband to wake up when in reality she has not moved. Her mind made her think that she was yelling but when she awoke she hadn't been, therefore there was no consequence for her action.