The question of whether or not mankind is alone in the universe is one of the oldest problems of philosophy, and has deep implications for our world view. In recent years, the subject has become increasingly important to science too. Advances in biochemistry and molecular biology have begun unravelling the mystery of the origin of life. Discoveries in astronomy are casting light on the existence of other planets and their chemical and physical make-up, while the space programme has provided the opportunity to search for life directly on our neighbouring planets. In addition, a major new project has begun which sets as its goal the detection of radio signals from advanced technological communities elsewhere in the galaxy. It is therefore very timely to consider in detail what the discovery of extraterrestrial life would mean for our view of ourselves and our place in the cosmos.
I make no attempt at a complete survey of the subjects. of exobiology, or the SETI programme as such (SETI stands for Search for Extraterrestrial Intelligence), as these have already been fully explained in many books. Instead, my concern is with the philosophical assumptions that underlie the belief in, and search for, life beyond the Earth, and