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Obviously, we cannot show in every case that a purported UFO was not an extraterrestrial spaceship. The data may not be available, and the events cannot be repeated. Furthermore, in several instances, very detailed data do exist in relation to a sighting, and yet it cannot be rigorously stated that the UFO Nevertheless, this approach has been followed in an attempt to eliminate this hypothesis, notably by Professor Donald Menzel, Director of the Harvard Observatory (Menzel and Boyd, 1963) and by the United States Air Force. Menzel is aware of the logical limitations, but he takes a statistical approach. He reasons that since many sightings can be positively eliminated as extraterrestrial spaceships, those which cannot could be if only more data were available. This is an excellent example of the inductive form of reasoning which has been so productive in science. Can we confidently apply it in relation to the UFO phenomena? To do so, the cases for which ample data exist and which prove not to be spaceships must be representative of the class as a whole. To many of us this seems unlikely, since other cases fortified with considerable data cannot be eliminated as extraterrestrial machines, and in many ways they appear to have little in common with the cases which can. On purely formal grounds, then, we cannot be absolutely convinced by Menzel's approach. It is also logically unreasonable to say with absolute certainty that it is impossible for extraterrestrial beings to visit us. Although we know a great deal about the universe, we do not yet know enough to make such an all-inclusive negative statement. Nevertheless, many of the arguments are highly compelling, and two are especially worthy of our attention. The first argument is that the UFOs contravene the laws of nature, or more properly, that they are contrary to our experience. It is first assumed that they could not originate within our solar system because only the Earth harbors intelligent life, and then it is reasoned that because of the extreme distances between stars they could not be visitors from some other planetary system. My initial contact with the UFO problem came because of my doubts in relation to the first assumption (Salisbury, 1962; 1964; 1966). Certainly we have no conclusive or even compelling evidence that Mars might support an intelligent civilization. We do, however, have a number of observations which seem to be in agreement with this assumption. The network of lines referred to as the canals still defies explanation in terms of non- intelligent phenomena, although such an explanation may well be apparent when we obtain more data about Mars. The satellites of Mars, with their almost perfectly circular, equatorial orbits and their small size have certain of the characteristics of artificial satellites. Brilliant flares of light occasionally seen on the surface of Mars are too short in duration and too bluish-white in spectral quality to be similar to our volcanoes, yet they are too long in duration to be readily explainable as meteorite impacts. An occasional associated white cloud would seem to eliminate them as reflections. It is even possible, if one is willing to stretch the imagination a bit, to find evidences for intelligence in the Mariner impossible for extraterrestrial beings to visit us. was not an extraterrestrial machine.