An infinite universe filled with life-sustaining planets is one result of the revolution begun by Copernicus and developed by his contemporaries. There is another implication of Copernicanism that deserves attention in a study of extraterrestrial civilizations. This is the introduction of the so-called Copernican principle, or principle of mediocrity.
The principle of mediocrity is a twentieth-century concept that grew out of the Copernican revolution of the sixteenth century. Although Copernicus never mentioned the principle in his works, it was derived from his Sun-centered model ofthe universe. Copernicus eliminated the Earth's special status as the center of the universe. The Earth was simply one of several planets revolving about the Sun. If the Earth is a planet, then the other planets are similar to the Earth.
According to modern interpreters of the principle of mediocrity, our region of the universe is typical of the rest of the universe. There is nothing special about the Earth. The sequence of chemical reactions that nurtured life early in the history of the Earth, and the biological and cultural evolution of terrestrial life, happened elsewhere in the universe, leading to similar results. The evolution of extraterrestrial life follows a predictable path. It produces intelligent creatures who develop technologies of travel and communications similar to those found on Earth.
The principle of mediocrity covers all aspects of extraterrestrial life and civilization. Virtually every commentator on the subject of extraterrestrial life, and every scientific research program seeking intelligent alien life, operates upon this principle. Therefore, it is important to remember that the principle is an assumption we make about the nature of unknown parts of the universe.
The principle of mediocrity may be a reasonable and useful assumption, but it is an assumption nevertheless. In essence, we assume that the rest of the universe is much like the locale in which we live. Those who believe that extraterrestrial life exists also suppose that its history mirrors the development of life on Earth. Carl Sagan, the twentieth century's leading advocate of intelligent extraterrestrial life, admitted that the application of the principle of mediocrity to unknown regions of the universe is "essentially an act of faith."3
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