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182 Other technological civilizations exist now, but they are separated from us by vast distances that make contact by interstellar communication or inter- stellar travel unlikely. This implies that other technological civilizations are relatively rare and that none have expanded throughout the Galaxy. Sagan and Shklovskii calculated that the average distance between tech- nical civilizations is between a few hundred light-years and about 1000 light-years; Asimov thought that the average separation might be as great as 600 light-years. The result may be that every civilization, no matter how far advanced, is isolated. As Shklovskii put it, we could be functionally alone.” Other technological civilizations do not perceive an external universe that could include inhabited worlds. Some extraterrestrials may be limited by their sensory abilities. Those living under densely clouded atmospheres might never detect their larger environments. Even where skies are clear, there may be no nearby astronomical bodies comparable to our Moon or the other planets of our solar system to stimulate the idea of other worlds. If intelligent aliens see only remote, stationary points of light in their skies, their relevant universes may be limited permanently to their own planet, sealed by a bowl of sky. Other civilizations have not passed through Copernican-level revolutions in placing themselves within the universe or among its living things. Few, if any, share our presumption that a multiplicity of inhabited worlds exists; even if they recognize the possible existence of other planets, alien tech- nological civilizations may not conceive of other worlds evolving life, intel- ligence, and technology. Our belief in unseen others may not be shared by all sentient beings; the fact that we imagine extraterrestrials does not guar- antee that they imagine us. We cannot assume that our level of curiosity is a universal characteristic of all civilizations. Some others—perhaps most—may never search their skies for evidence of alien minds. Other currently existing civilizations that have the scientific and techno- logical knowledge to search for signals, to transmit signals intended to attract the attention of others, or to send out interstellar explorers, do not use them. They may lack sufficient motivation to seek contact, or they may Reformulating the Problem Out of Range Failures of Perception Failures of Imagination Failures of Nerve