At some point I posted the Cosmos (original Carl Sagan version) episode dealing with Barney and Betty Hill and their claim of being abducted by aliens.
This is the episode in which Sagan stated that he would be happy if aliens really were visiting... but there isn't any evidence. This is where "extraordinary claims require extraordinary evidence" comes from.
Sagan's minimum threshold for acceptance was a physical artifact that could not have been produced on Earth, by humans (or any other lifeform native to Earth, I'm assuming).
If you subscribe to Michael Shermer's substack, he did an adaptation of Carl Sagan's "dragon in the garage" analogy today. I'll quote an excerpt though it's a public post:
“Yes, I most certainly would love to see your UFO!” you enthuse.
I eagerly oblige by taking you to my garage and opening the door. You look inside and see boxes of junk, empty cans and bottles, discarded tools and auto parts, a bicycle and assorted other nicknacks, but no UFO. “Where’s the alien spaceship?” you ask.
“Oh, uh, well, this spaceship has a cloaking device that renders it invisible,” I reply.
An “invisible UFO?” you query skeptically. Perhaps there’s a way to get around the spaceship’s stealthiness, you suggest, “by sprinkling flour on the floor of the garage so we could see the outline of the craft.”
“That won’t work,” I demure, “because this invisible UFO hovers a couple of feet above the ground.”
“Seriously? How does it do that?”
“My understanding is that it uses an anti-gravity propulsion system, and when it’s parked it is on idle and so it floats.”
“Um, interesting,” you acknowledge, then ask “what if we take a can of spray paint and spray all around the garage so we could at least see the body of the spaceship?”
“That won’t work, either” I explain, “because the energy from the cloaking device pushes all matter and energy—even light—out and around the spaceship, as if it were not even there, like those galactic-size lenses in space that bend the light from behind them. Maybe that’s where the aliens learned how to cloak their ships.”
“But,” you press on, your intrepid desire to test my claim leading you to think of other methods, “how about we get one of those portable thermometers everyone has been using during the pandemic and measure the temperature at different places in the garage to detect the heat from the UFO? Surely any cloaking technology and anti-gravity propulsion system would have to give off some heat energy, no?”
“Sorry,” I reply, “but I’m told that this spaceship operates at extremely low temperatures—remember, it traveled all the way to Earth from a distant planet through the coldness of space—so what little heat it gives off would be immeasurable by human-made thermometers.”
“Let me get this straight,” you groan in exasperation. “This UFO of yours is invisible, levitates, and gives off no heat whatsoever. Why should I believe you, then?”