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166 who accept what Brin called The Great Silence as evidence for humanity’s isolation. Both approaches, he commented, suffer greatly from personal bias and a lack of detailed comparative study.'” The easiest way out of the apparent paradox is to claim that intelligent extraterrestrials do not exist. That “solution” cannot be disproved unless we detect evidence of alien minds at work. Many of the participants in this debate reject that conclusion as wildly premature. The issue may be far more complicated than any one-factor analysis would suggest. Writing in the Quarterly Journal of the Royal Astronomical Society in 1975, Astronomer Michael Hart presented four explanations for there being no intelligent beings from outer space on Earth: A physical, astro- nomical, biological, or engineering difficulty makes interstellar travel infeasible; extraterrestrials have chosen not to visit Earth; they have arisen so recently that they have not had time to reach us yet; they have been on Earth, although we do not see them here now. Hart claimed that none of these explanations is convincing, leading to a conclusion that we are the first civilization in our Galaxy. Therefore, an extensive search for radio messages from other civilizations is probably a waste of time. Hart, who thought it likely that cultures descended directly from ours will occupy most of the habitable planets in our Galaxy, did admit that our descendants might eventually encounter a few advanced civilizations that never chose to engage in interstellar travel.’ Several people challenged Hart’s arguments, particularly the proposed rapidity of interstellar expansion. All reasonable combinations of assump- tions led to much slower rates than those Hart assumed, according to astronomer Laurence Cox of the Hatfield Polytechnic Observatory. To be universal, Hart’s theory must apply indefinitely to our own future; that, in turn, would require a fundamental change in our own society that renders ee nnt Linda + our past history useless for prediction. To astronomer George Abell, Hart’s analysis showed how easily we can use arguments concerning the characteristics of hypothetical civilizations to reach conclusions that we want to believe and thus how fragile our estimates of the number of other civilizations really are. Morrison thought arguments like Hart’s might be driven by a religious concern for demon- strating that we are the only conscious beings to be created." Other deniers pursued arguments similar to Hart’s. Basing their cases on population growth rates, they claimed that a relentlessly expanding Why Don’t We See Them? If They Could Expand, They Must Not Exist