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In 1981 , cosmologist Edward Harrison argued that such behavior would be an act of prudence : an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat .
 Edward Harrison
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:
 Edward Harrison