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Writing shortly before the discovery of hyperbolic geometry demonstrated the logical independence of the axiom - and long before the general theory of relativity revealed that it does not necessarily express a property of physical space - Schopenhauer criticized mathematicians for trying to use indirect concepts to prove what he held was directly evident from intuitive perception .
 Schopenhauer
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Schopenhauer