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In 1905 , Sigmund Freud described " sadism " and " masochism " in his " Three Essays on the Theory of Sexuality " as diseases developing from an incorrect development of the child psyche and laid the groundwork for the scientific perspective on the subject in the following decades .
 Sigmund Freud
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Sigmund Freud