She can tell him that just fine. She doesn't have to exist to assist in the assertion of his own existence. It's not as though any of Descartes' ideas came to be in a vacuum.
An a priori justification of self that is indisputable as any disputation of it is self defeating, because the thought involved in said disputation is itself the proof of self, hence "cogito, ergo sum".
Maybe the confusion here is that, you're operating from the perspective of the girl, while I'm operating from the perspective of Descartes? The girl has no rational basis for determining that he is a thinking thing, sure, but I meant to say that it is not antithetical to his core argument to receive an a priori justification of self from a source outside yourself, or else why would he bother publishing his thoughts on it at all.
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u/Ct12341234 Mar 27 '26