French thinker upsetting sacred set (9)

I believe the answer is:

descartes

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'french thinker' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'upsetting sacred set' is the wordplay.
'upsetting' indicates an anagram.
'sacred'+'set'='sacredset'
'sacredset' with letters rearranged gives 'DESCARTES'.

Can you help me to learn more?

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

I've seen this clue in The Guardian.

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