Page "Cogito ergo sum" Paragraph 3

A common mistake is that people take the statement as proof that they, as a human person, exist.

However, it is a severely limited conclusion that does nothing to prove that one's own body exists, let alone anything else that is perceived in the physical universe.

It only proves that one's consciousness exists ( that part of an individual that observes oneself doing the doubting ).

It does not rule out other possibilities, such as waking up to find oneself to be a butterfly who had dreamed of having lived a human life.