Why did Descartes pick thinking of all possible attributes to logically

Why did Descartes pick thinking of all possible attributes to logically establish existence? Rocks exist but don't think. What exactly did he have in mind to establish? Was it really existence? Did he have any valid reason to doubt his or our existence? Wouldn't pain be a better criterion? Or movement? Or change? If a non-philosopher raised such a question we would certainly look askance at him and not value his "evidence" either way.

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