While working on an essay for my English class I included this sentence:
The best solution is to take a page out of Cartesian theory and take a proven approach.
However, my teacher corrected it as follows:
The best solution is to take a page out of Cartesian's theory and take a proven approach.
However, this strikes me as wrong, "Cartesian" is not a person and this correction doesn't seem to make sense to me. I would understand if it was "Descartes' theory" as that would make it the theory of Descartes, but I don't understand how you can use a possessive with an adjective like in the correction.
My teacher hasn't been very helpful, her best explanation is "English is just like that". But I still would like to understand it, could someone explain it to me?
I also would like to know if there's a need to capitalize "Cartesian" in this case.