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The idea that humans are not alone in the Universe is ancient and lost in the mist of mythology. The myths of all ancient peoples are crowded with intelligent beings, often endowed with magical powers greater than those of humans. Often they dwell on Earth together with humankind or, if their abode is extraterrestrial, they live in a heaven that has little in common with the physical Universe. Actually, most ancient men and women could not distinguish between the physical Universe and the spiritual world. Astronomy dealt with fantastic creatures of all the types and, even in relatively recent times, the theological space occupied by God and the angels in Christian tradition often coincided with astronomical space.

Arthur C. Clarke wrote in 1951 in The Exploration of Space that there are people who are afraid that “the crossing of space, and above all the contact with intelligent but non-human races, may destroy the foundations of their religious faith.” He then went on to note that “in any event their attitude is one that does not bear logical examination — for a faith that cannot survive collision with the truth is not worth many regrets.”^1 Actually the idea that the discovery of extraterrestrial life, and above all intelligence, will give such a blow to religion to put all churches and religious institutions definitively “out of business,” as they say, is fairly common among atheist scientists. This idea is not new: in the era in which the plurality of the worlds was accepted by the majority of people (religious and not), Thomas Paine wrote in The Age of Reason (1794): “He who thinks he believes in both [the Christian view of the world and extraterrestrials] has thought but little of either.” And he would certainly choose to believe in extraterrestrials. Many SETI scientists would agree. Jill Tarter, for example, wrote: “God is our invention.⋯ If we get a message and it’s secular in nature, I think that says that they have no organized relion — that they have outgrown it.” And this will cause us to do the same.

Extraterrestrial life, its beginnings, and its evolution are the subject of astrobiology.^1 Some scientists, critical of this new science, expressed doubts about its scientific nature and defined it as the only science that does not have a subject, since there is no evidence that extraterrestrial life actually exists. This is in fact a groundless objection, since astrobiology studies the conditions needed for the development of life and tries to define general laws for evolution from the simplest to the most complex forms of life on the basis of the data that astronomy and astrophysics are slowly accumulating regarding the conditions prevailing on the various celestial bodies.

In its search for extraterrestrial life, astrobiology is limited by the enormous distances that separate the stars of our galaxy. As one may assume that interstellar probes will remain outside our possibilities for a long time, the only forms of extraterrestrial life we can discover outside our Solar System are those capable of being detected from a very large distance.

Every intelligent being interacts with other intelligent beings through his actions; these interactions are bound to cause advantages or damage (in terms of material or nonmaterial gains and losses) both for the agent and for its counterparts. An interesting method for evaluating human behavior, based on the gains and losses caused by these interactions, was proposed by Carlo Cipolla in his essay on the basic laws of human stupidity. The aim of the present interlude is to verify whether conclusions of the same type might be applied not only to humans of our planet but to all intelligent beings. The basic instrument is a plot of the type shown in Figure 1. On the horizontal axis are gains (losses are on the negative part of the axis) the actor causes to himself, while on the vertical axis the gains and losses he causes to the individual with whom he interacts. If the interactions are many, as happens when we want to study the behavior of a given actor with respect to many others, the average value of the gains and losses in the various interactions must be reported.

Some witnesses reported that in 1950 Enrico Fermi was discussing the possible existence of extraterrestrial intelligent beings with Edward Teller, Herbert York, and Emi Konopinski in Los Alamos. His opinion having been asked on the matter, he skeptically replied that he did not see any around. It sounds like just a joke, but actually it is a much more serious objection than may be apparent at first sight. Fermi was of the opinion that very long distance space flight will be possible (actually, he thought superluminal space travel was also likely), at least in the distant future, and that this possibility holds for any sufficiently technologically advanced civilization.^1 Since nobody has ever supplied convincing evidence of the presence of extraterrestrials on our planet, we must conclude that they do not exist.