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In a 2012 paper , economic theorist Perry Mehrling notes that what is commonly regarded as money can often be viewed as debt .
 Perry Mehrling
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Perry Mehrling