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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.

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    Your teacher is wrong. Yes, capitalize Cartesian.
    – SEL
    May 4, 2014 at 8:36
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    You're correct (if you're referring to Descartes, and not a character or organisation called 'Cartesian', both of which appear on the internet). English teachers are just like that. (Well, some of them.) While many 'proper adjectives' tend to lose their capitals after a few hundred years of constant use, I don't think 'Cartesian' has yet; check in dictionaries and Google searches.
    – Edwin Ashworth
    May 4, 2014 at 8:40
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    I feared that might be the case, but I wouldn't know how to explain why she is wrong. Could you please elaborate on why she is wrong?
    – user1090729
    May 4, 2014 at 8:41
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    Does your teacher also say that the Sherlock Holmes stories were set in Victorian's Era?
    – Eric Lippert
    May 4, 2014 at 16:19
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    In a mathematical context «cartesian product» arises constantly, and it is rarely capitalized.
    – Mariano Suárez-Álvarez
    May 5, 2014 at 4:47

3 Answers 3

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If you take a look at the Wikipedia disambiguation page for Cartesian (see http://en.wikipedia.org/wiki/Cartesian ), you will see that every expression in which the term appears relates to the French philosopher and mathematician René Descartes (the adjective 'Cartesian' being derived from the Latinized form of his name, Cartesius).

In my view, your teacher is not only ignorant, she is arrogantly ignorant: the link with Descartes ought to be part of every teacher's general knowledge, irrespective of their specialist subject area(s).

To answer your last question, 'Cartesian' does always begin with a capital letter.

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    ... though the insistance on capitalisation in this area could perhaps be regarded as quixotic.
    – Edwin Ashworth
    May 4, 2014 at 9:06
  • Thank you, would you say that the reason she is wrong is that "Cartesian" is not a noun but an adjective and as such it can't posses a theory?
    – user1090729
    May 4, 2014 at 9:27
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    @user1090729 Yes. It's the same situation as with "Newton" (noun) vs. "Newtonian" (adjective). We can talk about "Newtonian physics" or about "Newton's theories", but we certainly cannot talk about "Newtonian's theories".
    – senshin
    May 4, 2014 at 10:09
  • @EdwinAshworth: Wasn't he that Idiotic Donkey?
    – Martin F
    May 5, 2014 at 5:14
  • * martin f: You mean the asinine asinine?
    – Edwin Ashworth
    May 6, 2014 at 14:03
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'Cartesian' is an adjective derived from the proper noun 'Descartes'; the possessive form is only used with nouns. (And yes, you do capitalize it; the majority of adjectives derived from proper nouns are capitalized).

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As other posters have stated, your teacher is demonstrably wrong. It's difficult to imagine someone making such a basic error, but I would guess that she is unaware that "Cartesian" is the adjective form of "Descartes." She probably assumes that you are attempting to refer to a person named "Cartesian," and, since you are evidently in the process of learning English as a second language, further assumes that you have made a very basic grammatical error in the process.

Inform her that "Cartesian" is an adjective, not a proper noun.

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