Do I believe in UFOs or extraterrestrial visitors? Where shall I begin?

There's a fascinating frailty of the human mind that psychologists know all about, called "argument from ignorance." This is how it goes. Remember what the "U" stands for in "UFO"? You see lights flashing in the sky. You've never seen anything like this before and don't understand what it is. You say, "It's a UFO!" The "U" stands for "unidentified."

But then you say, "I don't know what it is; it must be aliens from outer space, visiting from another planet." The issue here is that if you don't know what something is, your interpretation of it should stop immediately. You don't then say it must be X or Y or Z. That's argument from ignorance. It's common. I'm not blaming anybody; it may relate to our burning need to manufacture answers because we feel uncomfortable about being steeped in ignorance.

But you can't be a scientist if you're uncomfortable with ignorance, because scientists live at the boundary between what is known and unknown in the cosmos. This is very different from the way journalists portray us. So many articles begin, "Scientists now have to go back to the drawing board." It's as though we're sitting in our offices, feet up on our desks-masters of the universe-and suddenly say, "Oops, somebody discovered something!" No. We're always at the drawing board. If you're not at the drawing board, you're not making discoveries. You're not a scientist; you're something else. The public, on the other hand, seems to demand conclusive explanations as they leap without hesitation from statements of abject ignorance to statements of absolute certainty.

Here's something else to consider. We know-not only from research experiments in psychology but also from the history of science-that the lowest form of evidence is eyewitness testimony. Which is scary, because in a court of law it's considered one of the highest forms of evidence.

Have you all played telephone? Everybody lines up; one person starts with a story and tells it to you; you hear it and then repeat it to the next person; the next person then passes it along. What happens by the time you get to the last person, who now retells the story to everybody who's heard it already? It's completely different, right? That's because the conveyance of information has relied on eyewitness testimony-or, in this case, earwitness testimony.

So it wouldn't matter if you saw a flying saucer. In science-even with something less controversial than alien visitors, and even if you're one of my fellow scientists-when you come into my lab and say, "You've got to believe me, I saw it," I'll say, "Go home. Come back when you have some kind of evidence other than your testimony."

Human perception is rife with ways of getting things wrong. We don't like to admit it, because we have a high opinion of our biology, but it's true. Here's an example: We've all seen drawings that create optical illusions. They're lots of fun, but they should actually be called "brain failures." That's what's happening-a failure of human perception. Show us a few clever drawings, and our brains can't figure out what's going on. We're poor data-taking devices. That's why we have science; that's why we have machines. Machines don't care what side of the bed they woke up on in the morning; they don't care what they said to their spouses that day; they don't care whether they had their morning caffeine. They're emotion-free data-takers. That's what they do.

Maybe you did see visitors from another part of the galaxy. I need more than your eyewitness testimony, though. And in modern times, I need more than a photograph. Today Photoshop software probably has a UFO button. I'm not saying we haven't been visited; I'm saying the evidence brought forth thus far does not satisfy the standards of evidence that any scientist would require for any other claim.

So here's what I recommend for the next time you're abducted into a flying saucer. You're there on the slab, where of course the aliens do their sex experiments on you, and they're poking you with their instruments. Here's what you do. Yell out to the alien who's probing you, "Hey! Look over there!" And when the alien looks over there, you quickly snatch something off his shelf-an ashtray, anything-put it in your pocket, and lie back down. Then when your encounter is over and done with, you come to my lab and say, "Look what I stole from the flying saucer!" Once you bring the gizmo to the lab, the issue is no longer about eyewitness testimony, because you'll have an object of alien manufacture-and anything you pull off a flying saucer that crossed the galaxy is bound to be interesting.

Even objects produced by our own culture are interesting-like my iPhone. Not long ago, the people in power might have resurrected the witch-burning laws had I pulled this thing out. So if we could get hold of some piece of technology that had crossed the galaxy, then we could have a conversation about UFOs and extraterrestrials. Go ahead, keep trying to find them; I won't stop you. But get ready for the night you'll be abducted, because when it happens, I'll want your evidence.

Many people, including all the amateur astronomers in the world, spend a lot of time looking up. We walk out of a building, we look up. Doesn't matter what's happening, we're looking up. Yet UFO sightings are not higher among amateur astronomers than they are among the general public. In fact, they're lower. Why is that so? Because we know sky phenomena. It's what we study.

One UFO sighting in Ohio was reported by a police officer. Some people think that if you're a sheriff or a pilot or a member of the military, your testimony is somehow better than that of the average person. But everyone's testimony is bad, because we're all human. This particular police officer was tracking a light that was darting back and forth in the sky. He was chasing it in his squad car. Later it turned out that the cop was chasing the planet Venus, and that he was driving on a curved road. He was so distracted by Venus that he wasn't even conscious of turning his steering wheel back and forth.

It's yet another reminder of how feeble our sensory organs are—especially when we're confronted with unfamiliar phenomena, let alone when we're trying to describe them.