Frenchman who wrote "Je pense, donc je suis” (9)

I believe the answer is:

descartes

(Other definitions for descartes that I've seen before include "Cogito ergo sum philosopher" , "Rene --, philosopher" , "French mathematician and philosopher" , "Cogito, ergo sum man" , "One who thought" .)

I've seen this clue in The Times.

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