Since the irrefutable fact remains that we dont know what we dont know, I wouldn't be so confident about knowing that kind of stuff, lest I seem naive, gullible, etc.
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So, then, why don't you apply that to the people who are claiming that there are alien spacecraft zipping about our planet, making all these physics-defying maneuvers?
If you're so afraid of appearing "naive, gullible, etc.", then agnosticism should be your default state. Simply refuse to come down on any side of the issue until there's virtually irrefutable proof one way or the other*.
Or, you could actually try to apply some
real skepticism to the various claims, and then arrive at a reasonable provisional conclusion. We have claims that UFOs are real, and claims that they are not. One position requires me to accept, as I pointed out in my original response, that just about everything we think we know about science in completely wrong. The other position requires me to believe that the human beings making these reports are some combination of mistaken, drunk or drugged, insane, or con men trying to make a buck off the previous three groups.
I have ample evidence that our science works - just take a look around you at the technology we've developed based on that science. If we've gotten the science that far wrong, it's a simply massive coincidence that we've gotten all the technology to work properly just by accident. So the conclusion that "UFOs are real" seems entirely unreasonable to me. It could still be true, but would require a mass of good evidence sufficient to overcome the weight of literally centuries of scientific and technological development.
I also have ample evidence that large numbers of people are one or more of mistaken, drunk or drugged, insane, or con men trying to make a buck. Arriving at the conclusion that UFOs are fake simply requires me to notice that lots of people will swear blind that homeopathy, psychics, Bigfoot, and good episodes of Season Three Star Trek are all real things. They clearly are not, and so coming to the conclusion that UFOs are not real is entirely reasonable.
*This is of course complicated by the fact that you can't prove a negative. There will never be "conclusive proof" of the non-existence of any of these claims, which means you'll never make a decision. As a pragmatic matter, at some point you'll have to make such a choice, at least for some things. When you're dying of cancer, will you do nothing, or make a choice about what kind of treatment to use?