Commented by Claus Larsen
The most famous UFO skeptic is Phillip Klass. He has written many books on UFOs as well as articles for aviation magazines such as Aviation Week. He is also one of the original founders of CSICOP, the Committee for the Scientific Investigation of Claims of the Paranormal.
UFOlogical principle 1
“Basically honest and intelligent persons who are suddenly exposed to a brief, unexpected event, especially one that involves an unfamiliar object, may be grossly inaccurate in trying to describe precisely what they have seen.”
Comment: Grasping to explain something that you have never seen or heard before can be quite a task. Think of how an Australian Aboriginee would describe a computer, if he saw it for the first time. If you didn’t know what a spaceship really looked like, how could you be expected to describe it correctly? Unless, of course, the spaceship looked peculiarly like those from out of Hollywood…
UFOlogical principle 2
“Despite the intrinsic limitations of human perception when exposed to brief, unexpected and unusual events, some details recalled by the observer may be reasonably accurate. The problem facing the UFO investigator is to try to distinguish between those details that are accurate and those that are grossly inaccurate. This may be impossible until the true identity of the UFO can be determined; in some cases this poses an insoluble problem.”
Comment: The problem with eye witnesses is that they often invent things that weren’t at the scene. This is not necessarily done to cheat, but most often to place the event in a context they are familiar with. If we only have the testimonies, it is virtually impossible to determine what really happened.
UFOlogical principle 3
“If a person observing an unusual or unfamiliar object concludes that it is probably a spaceship from another world, he can readily adduce that the object is reacting to his presence or actions, when in reality there is absolutely no cause-effect relationship.”
Comment: Magical thinking. A sorcerer sacrifices a goat to the Gods, and the rain comes. Step on a crack, break your mother’s back. Find a four-leaved clover, and you’ll have good luck.
UFOlogical principle 4
“News media that give great prominence to a UFO report when it is first received subsequently devote little, if any, space or time to reporting a prosaic explanation for the case after the facts are uncovered.”
Comment: The story gets the front page treatment, the retraction gets a short 50-word piece on page 16 – or any at all. We don’t hear about the dog who bites the man, but only about the man who bites the dog. Sensationalism is part of it, but also what captures our attention.
UFOlogical principle 5
“No human observer, including experienced flight crews, can accurately estimate either the distance/altitude or the size of an unfamiliar object in the sky, unless it is in very close proximity to a familiar object whose size or altitude is known.”
Comment: How big is the sphere in each of these three examples?
It depends on what we compare it to. In the first example, the sphere seems pretty small, where the second sphere could be huge and far away – or very small and very close. It is impossible to determine the size of the third sphere. 
UFOlogical principle 6
Once news coverage leads the public to believe that UFOs may be in the vicinity, there are numerous natural and man-made objects which, especially when seen at night, can take on unusual characteristics in the minds of hopeful viewers. Their UFO reports in turn add to the mass excitement, which encourages still more observers to watch for UFOs. This situation feeds upon itself until such time as the media lose interest in the subject, and then the “flap” quickly runs out of steam.
Comment: This is what noted author and editor of Skeptic Magazine, Michael Shermer, calls the Feedback Loop. Rumors start, then sightings are reported, someone gets blamed, usually the government. Denial of guilt is seen as proof of guilt. Then more and more reports come in, refuting the original rumors, authorities step in and the initial sightings are debunked. The system collapses.
UFOlogical principle 7
“In attempting to determine whether a UFO report is a hoax, an investigator should rely on physical evidence, or the lack of it where evidence should exist, and should not depend on character endorsements of the principals involved.”
Comment: Even the most elevated person can fail, either consciously or unconsciously.
UFOlogical principle 8
“The inability of even experienced investigators to fully and positively explain a UFO report for lack of sufficient information, even after a rigorous effort, does not really provide evidence to support the hypothesis that spaceships from other worlds are visiting the earth.”
Comment: This is the logical fallacy “Appeal to ignorance”, mentioned by Carl Sagan in his book “The Demon-Haunted World”. Absence of evidence is not evidence of absence.
UFOlogical principle 9
“When a light is sighted in the night skies that is believed to be a UFO and this is reported to a radar operator, who is asked to search his scope for an unknown target, almost invariably an “unknown” target will be found. Conversely, if an unusual target is spotted on a radarscope at night that is suspected of being a UFO, and an observer is dispatched or asked to search for a light in the night sky, almost invariably a visual sighting will be made.”
Comment: If you look for signs of witchcraft on a person (e.g. special markings, or numb spots), you are almost bound to find them. If you look for significance in tea leaves, or a horoscope, you will find it. We are pattern seeking animals, but sometimes, we see patterns that are not really there.
UFOlogical principle 10
“Many UFO cases seem puzzling and unexplainable simply because case investigators have failed to devote a sufficient rigorous effort to the investigation.”
Comment: It can often be very tempting to jump to conclusions, especially if you have a pet theory.
Source: The Mammoth Encyclopedia of Extraterrestrial Encounters, Ronald D. Story (Ed.), Publisher: Robinson.
