The most common and most likely answer to this crossword clue is the 9 letter word CARTESIAN
I've seen this crossword clue in The Guardian
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More clues leading to the result CARTESIAN
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The straight part
We know that "of a philosopher" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of a philosopher = cartesian.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CARTESIAN mean?
CARTESIAN adjective- Of, or pertaining to, Descartes, his mathematical methods, or his philosophy, especially with regard to its emphasis on logical analysis and its mechanistic interpretation of physical nature.
- Of, or pertaining to, co-ordinates based on mutually orthogonal axes.