The Great Filter is a proposed reframing of the Fermi Paradox, introduced by Robin Hanson in his 1998 essay The Great Filter - Are We Almost Past It?.
The development of space-framing intelligent life requires many steps to occur in sequence, such as the emergence of single-celled life and the transition from unicellular to multicellular life forms. Since we have not observed intelligent life beyond our planet, Hanson argues that there seems to be a developmental step that is so difficult and unlikely that it "filters out" nearly all civilizations before they can reach a space-faring stage — a "great filter".
From Hanson's essay:...