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He also wrote an introductory book on the philosophy of David Hume and a short biography of Voltaire .
 person: David Hume; person: Voltaire
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 person: David Hume; person: Voltaire