All skeptics have heard this from someone at some point in a debate: “You need to be more open-minded” or “You’re too closed-minded”. This is presented as though it is actually a valid argument. In reality it just shows they have run out of arguments. They hide behind it to disguise the complete lack of any rational reason for you to accept what they are telling you. It's the last resort of someone who has nothing – if they had evidence they would obviously present it.
Even so, it can seem compelling, since calling someone closed-minded is pejorative. But it’s fallacious rhetoric: doubting something is not necessarily closed minded. In fact, the closed minded ones are the believers who insist some fantastic story is true despite a complete lack of evidence to support it. They are too closed minded to accept that their fantasy might be false.
An open mind is…
Here’s the thing. An open mind is open to all ideas, but it must be open to the possibility that the idea could be true or false. It is not closed-minded to reject claims that make no sense. If you can’t accept the possibility that an idea might be false, then you are the closed minded one. An open minded person will critically examine all claims but will not accept them if there is no reason to believe they are true or if there is reason to believe they are false. To do so would be fallacious. And credulous.
The real problem
The “have an open mind” crowd are more than just logically wrong. Their way of thinking is actually destructive to good ideas. Bad ideas should be discarded - by weeding out bad ideas the good can flourish. An earlier version of this argument would have gone, “You’re closed-minded in saying that humors don’t exist” to justify bloodletting. But by focusing uncritically on bloodletting, germ theory would never have been discovered. Germ theory was discovered by skeptical scientists who insisted on evidence, not by new-agers with open minds.
If you accept something when there is no reason to believe it is true you are just credulous. And if you will not reject something when there is no reason to believe in it then you are in freefall – you will believe in anything. This way of thinking is a complete dead end. Or to put it another way, don’t be so open minded that your brains fall out.
That’s better – my mind is now completely open!
UK Skeptics have a good write up on this fallacy.
To see the fallacy in action, read some of the comments to my review of What the (bleep) do we know!? Start with this comment, then read this comment (and the rebuttals that follow them), but especially see this series of comments from Boney – see he stretches the have an open mind fallacy to amazing lengths.
Also, see this thread on JREF (long)