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The End of Eternity by Isaac Asimov is a science fiction novel, with mystery and thriller elements, on the subjects of time travel and social engineering. The themes are very different from most of his robot and 'space opera' stories, and take a clever approach to time paradoxes.
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Extraterrestrial Civilizations is a book written by Isaac Asimov in 1979, wherein the probability of there being intelligent extraterrestrial civilizations within the Milky Way galaxy is estimated. This estimation is approached by progressively analyzing the requirements for life to exist.
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