Philosopher and some things to mark his birthday? (9)

I believe the answer is:

descartes

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'philosopher' is the definition.
(I know that Descartes is a philosopher)

I can't explain the rest of the clue.

(Other definitions for descartes that I've seen before include "Cogito ergo sum philosopher" , "French philosopher, d. 1650" , "Rene --, philosopher" , "French mathematician and philosopher" , "One who thought" .)

I've seen this clue in The Times.

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