From what idea or disposition did the philosopher René Descartes derive his dictum "Cogito, ergo sum" ("I think, therefore I am")?

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René Descartes derived his dictum "Cogito, ergo sum" ("I think, therefore I am") from the effort to doubt everything. In his Meditations on First Philosophy, Descartes sets out to doubt everything he believes to be true in order to find a foundation of knowledge that cannot be doubted. The one thing that cannot be doubted, he believes, is that he is doubting, and therefore thinking, which leads him to the conclusion that he exists.

From What Idea or Disposition Did the Philosopher René Descartes