Not to downplay Descartes (his philosophy, specifically the process he undertakes, is worth a study), but something that the pop summaries of his writing really gloss over is that his entire premise hinges on something he takes as assumed true that need not be, and if it isn't, the rest goes straight off the rails.

Paraphrasing from memory: he pins "cogito ergo sum" on the assumption that objective knowledge is possible. That assumption is built atop imagining an alternative: an evil demon has full control over his senses and feeds him whatever it wants to (essentially, the 'Matrix hypothesis'). His solution to this concern? "A loving God wouldn't create a universe that banal and meaningless."

It's only a proof if you accept the postulate that objective reality is correlated with experience.