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As well as his contacts with people mentioned elsewhere in the article , he also maintained correspondence with Henri Bergson , William M. Brown , Martin Buber , George S. Counts , William Rainey Harper , Sidney Hook , and George Santayana .
 Henri Bergson, William M. Brown, Martin Buber, George S. Counts, William Rainey Harper, Sidney Hook, George Santayana
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Henri Bergson, William M. Brown, Martin Buber, George S. Counts, William Rainey Harper, Sidney Hook, George Santayana