Originally Posted by dyna mo

Fermi does and he illustrates that in the Fermi Paradox
http://www.seti.org/seti-institute/p.../fermi-paradox

the drake equation also
http://www.seti.org/drakeequation

and the great filter, The Great Filter, in the context of the Fermi paradox, is whatever prevents "dead matter" from giving rise, in time, to "expanding lasting life". The concept originates in Robin Hanson's argument that the failure to find any extraterrestrial civilizations in the observable universe implies the possibility something is wrong with one or more of the arguments from various scientific disciplines that the appearance of advanced intelligent life is probable; this observation is conceptualized in terms of a "Great Filter" which acts to reduce the great number of sites where intelligent life might arise to the tiny number of intelligent species actually observed (currently just one: human).