Just for a moment, assume that there really are aliens out there and some of them are flying spacecraft around, maybe scouting the earth for the prospects of future colonization. And you see one. And the government knows you saw one. It knows you are a witness.
Why would the government need to cover up the possible existence of UFO's? Go into work tomorrow and tell everyone in the office that you saw a UFO last night. What do you think? You are now the best friend of any government that wants to convince people not to take UFO sightings seriously. You can't help but acknowledge the inherent absurdity of what you are saying, and the more you know that, the more you are likely to giggle, or become demonstrative or excited, which makes you look crazy, of course.
Suppose instead that you don't believe in UFOs and then you see one. You would not have seen one, because you will not believe what you have seen. Contrary to the movies, in which spontaneous revelation happens all the time, you will simply not believe what you have seen. You will tell people, I saw something that looked almost like a UFO. But actually, it was obviously some kind of civilian aircraft, or a helicopter, or an optical illusion. It couldn't have been a UFO.
Even if you could produce a UFO, you would explain that it is some kind of advanced experimental aircraft, probably, that the government is trying to keep secret. With hairless bulbous monkeys inside.
There is absolutely no proof that UFO's do not exist. There is a lot of proof that no matter what people actually see or hear, certain smart people will never, ever believe it was a genuine UFO.
I am thinking of three cases in particular. The sightings over Shag Harbour, Nova Scotia on October 4, 1967; the one over Lebanon, Illinois on January 5, 2000, and the one near Kecksburg, Pennsylvania, December 9, 1965.
In each case, numerous reasonable and otherwise reliable individuals reported seeing an object in the sky that did not appear to resemble any other known type of aircraft. In each case, you have to grant the possibility of numerous whacky people all agreeing to tell ridiculous stories to the media for personal reasons we will never understand.
All right. I realized as soon as I typed "numerous reasonable and reliable" that maybe I'm asking too much of any random sampling of individuals who happen to be in the same general area at any given time. And at different periods of history, "numerous reasonable and reliable" people concluded that there were witches, communists, and terrorists everywhere around us. "Numerous reasonable and reliable" people think that nuclear power plants are less safe than coal-fired power plants. "Numerous reasonable and reliable" people think that ethanol can help save us from the energy crisis. Numerous "reasonable and reliable" people think the Leafs have a shot at making the playoffs.