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For those with difficulty with the concept of something being possible but not true , the meaning of these terms may be made more comprehensible by thinking of multiple " possible worlds " ( in the sense of Leibniz ) or " alternate universes " ; something " necessary " is true in all possible worlds , something " possible " is true in at least one possible world .
 Leibniz
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:
 Leibniz