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Their metaphysical status has been a subject of controversy in philosophy , with modal realists such as David Lewis arguing that they are literally existing alternate realities , and others such as Robert Stalnaker arguing that they are not .
 David Lewis, Robert Stalnaker
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:
 David Lewis, Robert Stalnaker