The simple fact is -- that there are no UFOs, at least that is the fact as dictated by many government agencies and understandably skeptical technical people. Many blue collar beer barrels simply assume the whole subject is just so much "[expletive deleted], as well."
But these are not the people who have seen them. Surely, a person might still perceive a mystery craft in the sky where there is nothing. The mind may construct optical illusions or hallucinate things and the person does not need to be mad or flawed in any way. Just looking at the sky can cause the eye to reveal all kinds of little things that go unnoticed during normal daily viewing.
But the devices may actually be real. Then one of the mystery things can be either some normal device that is mistaken as to shape or purpose, including those famous "Venus as a flying saucer" sightings. The Moon has also caused many people to be convinced that a machine of some kind was following them.
The parallax effect of vision is obvious to some people but not necessarily to everyone. Any bright star or planet in the sky can then be mistaken to possess a "following" motion. This is simply the effect of far distant objects having less apparent motion than closer objects. The bushes beside the highway seem to rush by whereas the distant mountains pass much more slowly.
There may be, however, actual devices in the sky other than objects in space that cause the sightings, whether flying saucers, airplanes or helicopters.
There are also experimental military machines, or at least test machines of SOME kind. Even unclassified industrial experiments might be held in secret in order to lessen the knowledge gained by competitors. The military is more secretive, of course, and once things sink into the military industrial complex they become mired in thick layers of obfuscation. Even unnecessary things become covered up by military personnel simply to avoid responsibility for anything whatsoever that might go wrong. Order too many boxes of toilet paper? Cover it up.
Now there still exist the possibility of exotic machines that lie outside all the above attempts at explaining them. But since there is always the probability of error in every report of such machines (unless they just stop and allow people to crawl all over them, taking copious pictures), there can never be absolute scientific proof. It might be 50% true, or even 90% true, but never 100% true. This is often the case in legal cases, both criminal and civil, although it would seem that 100% truth would be preferable in death penalty cases.
To a person who has seen them (and whom I have talked with directly,) there is none of this wishy-washy explaining away of things. They saw a machine which was obviously metallic, with glass windows or portals, and it was able to hover and move in ways no regular aircraft can move. This does not mean they are "alien space ships" or whatever, only that they are not obviously any known kind of machine within that witness's knowledge base.
The people who claim that the creatures flying these machines are alien might be a bit overreaching or they might actually be telling the truth, although I have no way to prove them wrong, nor are they able to prove themselves right. There are many cases of sightings and they might range from "automatic electronics set on autopilot" to "humanoids from The Xanadu Galaxy." I don't know what is really true about that but I would bet money that it is not the latter.
But I will listen to whoever has anything to say about these things, without jeering, pointing fingers, hurling derision or inflicting humiliation. Nothing of it has been proven conclusively, true or false, although much has certainly been debunked. (Swamp gas IS NOT a substantial substitute for a real explanation!)
The burden of proof is still on the witness in these cases, yes, but one cannot prove a negative, either. It is much like the Atheists versus the Theists. Both are believers in something which cannot be proven. Yet, if God decided to talk to me, I'd listen. Even if it was all in my mind.