Descartes & Marcel answer the question, “Who am I?” differently. What accounts for such different discoveries?

Question

Descartes: Cogito, ergo sum. I think therefore I am.

[I]f I judge that the earth exists from the fact that I touch it or see it, this very fact undoubtedly gives even greater support for the judgment that my mind exists.

Marcel: When I say, not that I am, but that I exist

“My body.” My body, insofar as it is my body, is both something that I have and something that I am, and cannot be adequately accounted for using either of these descriptions alone.

Answer 1

The 300-year gap between their writings has a lot to do with it. Huge differences in the scientific background they were writing against. That is probably the biggest difference.

Plus, Marcel's point is sort of a conclusion, while Descartes' oft-quoted phrase is only the start of an argument, in the course of which (In Discourse on the Method of Rightly Conducting the Reason) he gradually works his way round to concluding that his body - as well as everything else - necessarily exists. So chances are he would not disagree all that much with Marcel anyway.

Answer 2

There's actually a very prominent philosopher who teaches at Tufts University in Medford,MA, Daniel Dennett, who wrote a critique and revision of Descartes' 'dualistic' explanation of consciousness and existence. Dennett describes the 'Cartesian Theater' as hogwash and hopeful child's play: there is no such thing as a 'self' separate of the body; there is no transcending of an immortal soul because 'consciousness of self' (and he goes so far as to completely explain consciousness away) is an illusion. Basically, your mind comes with your brain. When your brain ceases to function, you lose your mind, your sense of self, and any indication of "your" existence.  

Answer 3

for each person, having grown up with different experiences in life, they will come up with a different answer than someone else will, depending on how they grew up and what happened in their life to make them think that way.

Answer 4

They actually aren't that different at all.

It's just semantics.

And when Descartes said "Cogito, ergo sum" I don't think he really meant it. I think what he probably meant was that questions about the afterlife and what our purpose is are fruitless and questions about what existence is are fruitless (like the whole "are we in a matrix?" deal) because we experience things all the same.