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169 Tipler’s Historical Perspective In 1981, Tipler presented a capsule history of the extraterrestrial intel- ligence concept in the Quarterly Journal of the Royal Astronomical Society. As he saw it, the debate reoccurred periodically as the centuries passed, with new generations of debaters rediscovering pro and con arguments that had been used earlier. He concluded that, as has been the case for 2000 years, philosophical and theological beliefs are the main motivations for the belief in extraterrestrial intelligence.”* According to Tipler, believers in alien intelligence have tended to lack what he called a sense for the contingency of history and its unique, unpredictable events. If we apply contingency to expansion, we must question the inevitability that underlies Tipler’s theory of self-reproduc- ing probes. Technological civilizations may or may not choose to begin colonization; they may or may not choose to continue it. Tipler assumed that a particular technological model would be maintained for millions of years, although our own history suggests that such concepts have much shorter lifetimes. cian and cosmologist John Barrow. According to these authors, an intelli- gent species with the technology for interstellar communication would necessarily develop the technology for interstellar travel, and this would automatically lead to the exploration and/or colonization of our Galaxy in less than 300 million years (emphasis added). Von Neumann machines would be programmed to turn some of the material in the alien system into an O’Neill-type colony; those machines would synthesize the colony’s inhabitants from genetic information. Tipler’s argument rested on the assumption that any intelligent species would expand into new environments, launching Von Neumann probes or colonization ventures of some type. If we deny this assumption, he acknowl- edged, we have nothing to go on except opinion.** Drake described the basic weakness of arguments such as Tipler’s: the readiness to assume the possible to be the inevitable. Advanced extrater- restrials may not want to spend the money and the energy to attempt interstellar travel. They may see no personal gain in creating a costly army of Von Neumann machines; they may be content to colonize their own star system; they, like us, may have found radio communication the more prom- ising alternative.*° To Finney, the central premise for Tiplerian technological optimism is that whatever is not forbidden by natural law will come to pass. From this Counter Arguments Tipler restated his case in a 1986 book he co-authored with mathemati- cae ned weed Tab Dane Aad be the eb ne tee Counter Arguments