The concept of extraterrestrial intelligence is a worldview of its own, a "biophysical cosmology" that seeks confirmation no less than physical views of the universe. It is, however, a subject at the very limits of science, and scientific attempts at confirmation therefore illuminate the nature of science itself. This history is not only important for an understanding of the nature of science, but is also central to any forward-looking concept of religion, philosophy, and numerous other areas of human endeavor. Extraterrestrial life will be one of the predominant themes of science in the 21st century.