I am away this week filming the pilot for The Skeptologists. For NeuroLogica this week I am updating and editing some previous essays that I have written. This one was originally published in my Weird Science column in November 2004.

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I listened patiently as the UFO enthusiast explained how humans were transplanted to the earth from another world by our alien forebears.

“Then how do you explain the fact that humans share 98% of our DNA with chimpanzees, and a genetic code will all life on earth?” I asked.

“Well, I think you have to keep an open mind,” was her starry-eyed response.

People who believe odd things-that coffee enemas can cure cancer, that the Loch Ness Monster exists, that art on the dollar bill can explain secret conspiracies-are always telling us to “keep an open mind.”

The now standard skeptical response is to quip: “Keep an open mind, sure–but not so open that your brains fall out.” In the endless sparring between skeptics and believers, the “keep an open mind” rejoinder is the favorite weapon in the believers’ arsenal. It is their all-purpose tool. But what does it really mean to be open-minded, and is it the skeptics or the believers who are truly closed-minded?

Having an open mind is a curious virtue: Everyone agrees that open-mindedness is indeed a virtuous state, but it’s most often praised by those being decidedly closed-minded. Also, the most open-minded people are those whom you would least suspect as paragons of this particular virtue – skeptics. And those most lacking open-mindedness are those most likely to admonish others for not being so – true believers.

Let me explain.

Having an open mind means you don’t dismiss claims to truth out of hand. You analyze first. When you analyze a claim, you consider all the relevant evidence and examine all the logic involved, in a fair and unbiased manner, then grant tentative acceptance or rejection. If new arguments or new evidence come up, then you revise your opinion. Being open means that you apply this standard fairly to all claims. Being open-minded does not mean believing every claim, no matter how improbable-that’s being gullible, not open.

This process of fair analysis, based upon logic and evidence, leading to tentative conclusions, which are open to revision, is part of science. It is also the very soul of true open-mindedness.

By contrast, true believers adhere to a desired claim regardless of evidence or logic. No argument is persuasive enough, and no evidence (or lack thereof) is compelling enough to nudge them from the perch of their belief. They are closed to the possibility that Bigfoot might be a delusion, that crop-circles are pranks, that coffee enemas don’t cure cancer-that their cherished belief might be wrong. Yet it is such believers who most frequently claim the moral high ground of open-mindedness, then condemn unbelievers for being closed-minded. They wish others to accept their claims either without examining the logic and evidence, or despite refutation from such examination.

The “open-minded” police often use the label of “closed-minded” as a personal attack to dismiss the arguments of those who dare to examine their claims (and are therefore committing an ad hominem logical fallacy). You do not believe I was abducted by aliens, they argue, because you are closed-minded. (Could it be because they lack credibility or any evidence to back up the claim? Or because the claim is inherently improbable?) To them, being abducted by aliens is an article of faith, much as religious believers take certain beliefs on faith.

That’s fine. People are entitled to their faith. It’s an important freedom, guaranteed in the Constitution. But personal faith cannot be used to justify a scientific claim about the factual state of nature. Scientific claims must be public, open, and transparent-they cannot be based upon secret knowledge, special talents, or unquestioned virtues. If you think aliens have visited the planet, you must be prepared to offer evidence, not just accuse other people of being closed-minded for not believing you.

Science is also a cumulative process. At this point in history we happen to be sitting on centuries of painstakingly accumulated scientific knowledge. It would be both hubris and folly to ignore all that has gone before. So while keeping an open mind to new ideas and claims, it is to our advantage to view such claims through the filter of established knowledge. Believers would have us view new claims in an intellectual vacuum, as if all claims were inherently equal.

So let me turn the tables and humbly ask you, dear reader, to be truly open-minded. Adhere to the advice of David Hume, who wrote, “A wise man proportions his belief to the evidence.” Keep your brains tucked safely inside your skulls. Be open but not gullible. And remember, it is better to think than to believe.