Jawaan!

The Great Filter is the idea that there is some single, almost insurmountably improbable barrier on the path to the stars that explains why we've never seen any sign of alien life.  It combines aspects of astronomy, biology and history to arrive at one inescapable conclusion: university professors dream of book deals. Robin Hanson of George Mason University posits a “Great Filter” that prevents the rise of intelligent, self-aware, technologically advanced, space-colonizing civilizations. The “filter” would be one or more improbable steps along the path that starts with the creation of a planet and ends with a race capable of colonizing the galaxy

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"The Great Filter": Science Fact or Fiction? -A Galaxy Classic