UFO stands for unidentified flying object, and no one would deny that there are flying objects which haven't been identified. The argument here is against is the idea that some UFOs are the spacecraft of visiting aliens (or other paranormal phenomena such as psychic energy events, etc) because this is the idea most people associate with UFOs (see note 1).
UFO reports exist in ancient texts such as the Book of Enoch which describes flying objects and possibly alien beings. It even says that Enoch was taken on journeys to various parts of the Earth and even to the heavens (see note 2).
Many UFO theorists see the existence of alien visitors as a convenient way to explain various phenomena which they believe can't be unexplained otherwise. For example, one web site says this "...it is still not known how humans originated on this planet. The Darwinian theory of evolution remains a fascinating, yet unproved, hypothesis, simply because the elusive 'missing link' remains undiscovered. The alleged link between humans and their anthropoid cousins may have been provided by visitors from another world." (see note 3). Any site which rejects evolution as the likely origin of modern humans and which claims no missing links exist should be treated with great skepticism. We should follow Occam's Razor (see note 4) and accept the simplest explanation: evolution is well proven and tested, UFOs have far less credibility (see note 5).
Probably the most famous UFO case is the Roswell Incident (see note 6). This involved the alleged recovery of an extra-terrestrial spacecraft, including alien bodies, from an object that crashed near Roswell, New Mexico, in 1947. The initial incident was largely ignored at the time but the popularity of the incident was boosted in 1978 when a ufologist, Stanton T. Friedman, interviewed a participant in the original recovery of the debris. It was then that the claim was made that the military had covered up the recovery of an alien spacecraft and it became very popular in the UFO community.
At that time the story escalated into one involving up to 11 crash sites, a huge military cover-up, including intimidation of witnesses, and even autopsies carried out on alien bodies (see note 7). The US government responded with an investigation which concluded (in 1995) that the stories were based on a crashed balloon from a secret government program called Project Mogul (this used high altitude balloons to detect sound waves generated by Soviet atomic bomb tests and missiles.) A second report, released in 1997, concluded that reports of recovered alien bodies were probably confused memories of military accidents involving human casualties (see note 8), as well as hoaxes.
Today even many UFO supporters agree the Roswell incident is unlikely to have involved alien visitors (see note 9). Again Occam's Razor should be invoked. Instead of alien visitors crashing the more prosaic explanation, a military accident is a far better conclusion to reach. So the most spectacular and widely cited case of UFO visitors is easily dismissed. That doesn't mean the others aren't true but it does show how a simple incident can be exaggerated far beyond what really happened and the truth is only revealed when a thorough investigation is done. Of course it could be a huge government conspiracy but the problem with conspiracy theories is that they can never be disproved because contrary evidence is just seen as part of the conspiracy!
Explanations of UFO Sightings
Balloons, especially high altitude weather balloons, are a common cause of UFO sightings, but there are many other natural and well documented phenomena which are also often confused with alien spacecraft. I'll present a few here but there are many more which could be included (see note 10).
The Moon, Sun, and planets, especially when viewed in unusual circumstances often lead to UFO sightings. When viewed from a moving vehicle the Moon often seems to be following the person making the report. Venus is usually low in the sky and bright. It often undergoes scintillation leading to apparent coloured flashing lights. The light from the Sun can reflect from ice crystals leading to bright lights in unusual parts of the sky. It's interesting that very few astronomers report UFOs. They spend a lot of time looking at the sky so we should expect a lot of reports from them. Is this because they are part of the conspiracy or do they just know what they are really looking at? The simplest answer is obvious!
Conventional and experimental aircraft are another common cause of sightings. Aircraft flying at high altitude are often silent (because of their distance), sometimes seem to stay motionless (again because of their height) and often leave unusual vapour trails behind them. All of these are typical of UFO sightings. Military operations often involve fast maneuvers, flares, missiles, and secret plans which might be denied later. All of these are great candidates for UFO reports.
Natural phenomena such as clouds sometimes assume unusual shapes which can be confused with solid spacecraft. A favourite are lenticular clouds (see note 11) such as the one shown in the picture at the top of this page.
The web site TopTenz (see note 12) lists these as the 10 most common causes of UFO sightings: the planet Venus; meteors, comets, and fireballs; lenticular clouds; ball lightning; weather balloons; satellites; flares; birds, lens flare, reflections, and other abnormalities; aircraft; and experimental military aircraft.
As if all of the possible natural explanations weren't a big enough problem, there is also the problem of hoaxes. On many occasions UFO enthusiasts have been guilty of faking photos and movies (see note 13) of alleged flying saucers. Some are crude and obviously faked but others are quite sophisticated. These can be quite difficult to debunk. There is also the additional issue of people with psychological problems who imagine they have been abducted and abused in various ways by aliens. These are rarely supported by any physical evidence.
Argument from Ignorance
Finally, there is the point that an unidentified object should not be automatically assumed to be an alien visitor. There are many UFO cases which haven't been explained. Often this is because the evidence is lacking, the report is from personal experience with no physical evidence, the incident happened in the past, or the evidence is of poor quality (bad photos, fuzzy video, etc). But if there is no explanation we should say so. The object really is an unidentified flying object, but we should not assume it's from another planet!
So in summary there are many cases of unidentified flying objects that there is no good conventional explanation for, but when cases (even those which seem convincing at first) are examined explanations tend to be found. It is possible (in fact it's likely) that alien life exists elsewhere in the universe, and its possible (but maybe unlikely) that it has visited Earth, but it's unlikely that the UFO reports we are seeing now are anything except standard phenomena being misinterpreted.
While its almost impossible to believe that we are alone in the Universe, there is still no credible evidence that UFOs are alien visitors from other planets, although the idea of alien visitors is quite credible in principle. Therefore I give this a moderate score on the crap-ometer!
1. A good overview of the UFO phenomenon is available at the Wikipedia, UFOs web page.
2. There is an extensive discussion of the Book of Enoch at Wikipedia, Book of Enoch which lists its complex history and translation. There are also links where you can find the actual text and further discussion.
3. A web site which makes various claims regarding UFO phenomena in ancient times and suggests they are a good explanation of unexplained phenomena is Unexplained Stuff, UFOs in Ancient Times.
4. Occam's Razor more accurately states that the best theory is the one which requires the least new assumptions. We already know evolution is real but there is no good evidence that alien visitors exist, therefore evolution is a better explanation. There's a good explanation at Skepdic.
5. Although deliberate manipulation of life by aliens leading to humans is unlikely the theory that primitive life may have travelled to Earth on meteors is taken quite seriously. Note that the credibility of the primitive life theory in no way leads to credibility to the alien visitor theory though.
6. The Roswell Incident is quite thoroughly covered (in a fairly neutral and fair way) at Wikipedia, Roswell Incident.
7. A film claiming to show actual footage of alien autopsies from Roswell was debunked in an article at a usually pro-UFO conspiracy web site, Cosmic Conspiracies, The Alien Autopsy.
8. At the risk of sounding like a conspiracy theorist myself I would like to point out that the military has remained silent regarding many cases where UFOs were blamed but were actually most likely the result of military aircraft activity.
9. William L. Moore, one of the earliest proponents of the Roswell incident, said this in 1997: "After deep and careful consideration of recent developments concerning Roswell... I am no longer of the opinion that the extraterrestrial explanation is the best explanation for this event." Moore was co-author of the first book on Roswell, The Roswell Incident.
10. Some of the most difficult to explain events involve personal encounters with aliens but these are often the result of sleep paralysis. There's a description of the phenomenon at Wikipedia, Sleep Paralysis.
11. Lenticular clouds are stationary lens-shaped clouds that form at high altitudes and are often mistaken for UFOs. There is a discussion at Wikipedia, Lenticular clouds.
12. I don't think the TopTenz site itself is necessary very credible (it lists no sources for example) but it does give the commonly known explanations at TopTenz, Top 10 Common Explanations for UFOs.
13. The Listverse site has a list of what the authors claim are the most famous hoaxes: Listverse, 10 Most Famous UFO Hoaxes. There is also an incomplete list at Wikipedia, List of UFO related hoaxes.
Sources of Further Information
There are many web sites which seem to offer convincing support for UFOs. But its important to look behind the scenes because there are mundane explanations for most of the best UFO cases. And even if there is no explanation its still wrong to jump to a conclusion involving an extraordinary explanation or conspiracy theories.
Wikipedia, UFOs A general overview of the subject of UFOs.