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Introduction speculations wrestled with another fundamental question. If there are intelligent beings beyond the Earth, some of them might be more intellec- tually advanced than we are. How, then, could Man be the measure of all things? The common thread, the underlying philosophical question, is the importance of Humankind. What is the status of humans in the cosmos? Are we unique, a chosen species, or are we but one of many examples of intelligent life? “The answer calibrates our place in the universe,” said astronomer Jill Tarter.* The public is engaged. Opinion surveys since the 1960s have shown a steady rise in the percentage of North Americans who believe that extra- terrestrial intelligence exists. A 2000 poll found that 82% of Americans thought there was intelligent life elsewhere in the universe.’ Many people in other countries share these views. The higher the education, income, and occupation level, the more likely a person is to believe in the existence of extraterrestrial intelligence. “What has to be explained is not that some people are interested in the subject,” said Carl Sagan, “but that some people profess not to be interested in it.” We are in the preconfirmation phase of this intellectual enterprise— the stage that allows the widest and most imaginative range of pos- sibilities. Thinking about intelligent life elsewhere stretches our imaginations. We must conceive of gulfs of space and eons of time. We must visualize alien environments, in which evolution could take very different paths. We must imagine the histories of civilizations unlike our own, projecting their evolution far beyond our present level. Wide-ranging speculations are not only possible, they are required. When we discuss extraterrestrial intelligence, we cannot limit ourselves to the narrow perspective of one science or academic discipline. Astronomy, biology, history, even climate studies contribute to the debate. As philosopher David Lamb saw it, no other subject is as capable of bringing together the fragmented elements of contemporary science.* This applies not only to scientists but also to nonexpert citizens, for whom this topic may be the most intriguing entry portal into science’s diverse attractions. Speculations about contact often challenge conventional wisdom, from biology to religion. Imagining extraterrestrials, and what they might do, irritates those most sure of an established worldview. We must reconsider what we have taken for granted. Our thinking about extraterrestrial intelligence also must contend with an innate human longing to feel important. To some people, that requires that we remain unique. The easiest solution available to them is to declare that intelligent aliens do not exist.