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168 Hart admitted that there could be a small number of other civilizations in our Galaxy, none of which were interested in interstellar exploration and colonization, or ever had been in all the ages since they first acquired the technological capability. He thought it more probable that the chance that any specific galaxy will contain life is extremely small. The reluctance to admit that the number is low is primarily a result of wishful thinking; a galaxy teeming with bizarre life forms sounds a lot more interesting than one in which we are alone." Papagiannis too found it extremely unlikely that a large number of long- lived civilizations could have existed in our Galaxy without any of them starting the colonization process. It is virtually impossible to find a univer- sal reason that would prevent each of them from initiating interstellar travel. (Papagiannis assumed a natural tendency for life to expand to occupy all available space.) There was no way that the colonization wave would have missed or by passed our solar system, which meets all of the essential requirements for the establishment of space colonies—longevity, stability, a suitable sun, and a multitude of planets, moons, comets, and asteroids. Like Kuiper and Morris, Papagiannis argued that one is led to two dia- metrically opposed alternatives. First, the Galaxy already has been colo- nized; if so, there must be space colonies in orbit around every well-behaved star. Second, the Galaxy has not been colonized because of the lack of a substantial number of advanced civilizations.” Papagiannis later revised his argument. Colonizing civilizations would not come to live on planets, as they would be accustomed to living in space colonies. They would build more of those colonies using the raw materials obtained from asteroids and small moons. We should search our asteroid belt before concluding that colonization of our solar system has not taken 21 place. Tiplerism The “Extraterrestrial Beings Do Not Exist” school found its most vocal champion in physicist Frank Tipler. His attack on SETI may have been motivated in part by what he called its semireligious overtones.” Tipler proposed in 1980 that advanced civilizations would use self-rep- licating probes (sometimes called Von Neumann machines) to explore or colonize the Galaxy in a relatively short time. If extraterrestrial intelli- gences existed, their spaceships must already be in our solar system. As we don’t see them, they do not exist. Interstellar travel would be simple and cheap for a civilization only slightly in advance of our own, Tipler claimed 3 years later. If such a civiliza- tion had ever existed in the Galaxy, their spaceships would be here. “There must be a first civilization,” he declared, “and it happens to be ours.” Why Don’t We See Them?