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Mental operations have been investigated at a developmental level by Jean Piaget , and from a psychometric perspective by J. P. Guilford .
 Jean Piaget, J. P. Guilford
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Jean Piaget, J. P. Guilford