The acronym UFO stands for Unidentified Flying Object. In general, this can mean anything from a slow-moving cloud in the sky to a fast-flying dish thrown by an angry wife. But when people ponder whether UFOs exist they mean the real thing: intelligent aliens that have arrived on Earth from another star system. It is in this context that I will use the term UFO here. Have UFOs visited us in the past? Are extraterrestrial aliens in our midst? Consistent with analysts who have tackled the issue of extraterrestrial life and visits, I will divide the question in parts. Take note, however, that knowing what intermediate questions to ask is a science unto itself. In my humble opinion, the following are the questions we should ask when pondering the popular topic of extraterrestrial visits:
• Is there life of any kind outside the Earth? • Does intelligence necessarily develop wherever life develops? • Does human-level intelligence or higher necessarily develop where the environmental conditions are Earth-like?
Only if we answer these questions affirmatively can we proceed to speculate about the aliens' physical appearance or about whether they have mastered the technology to travel to or communicate with Earth. Otherwise, we have no chance of ever meeting or communicating with extraterrestrial aliens other than in Hollywood studios or in the corridors of Cambridge and Harvard Physics Departments. These are the questions I intend to address here.