When Descartes says, "I think therefore I am", what sort of argument is he using

When Descartes says, "I think therefore I am", what sort of argument is he using

When Descartes says, "I think therefore I am", what sort of argument is he using? Is he simply saying, "There is an X, and X thinks, therefore there is an X", in which case he might as well have said "I walk therefore I am" or, with Barbara Kruger, "I shop therefore I am"? Or is the argument here more esoteric?

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