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prior plausibility

The concept of prior plausibility originated, as far as I know, in discussions over the role of plausibility and prior probability in medicine. The discussions were led by Steven Novella, Kimball Atwood, and others at Science-Based Medicine in their effort to distinguish science-based from evidence-based medicine. I discuss prior plausibility in medical research in the entry on science-based medicine, but here I want to explore the role this concept plays in skeptical investigations in general.

Many prominent skeptics have cautioned the rest of us not-so-prominent skeptics against debunking and have advised us to be open-minded in our investigations of paranormal and other questionable claims. I, on the other hand, while never being an opponent of open-minded inquiry, consider some claims to have near zero prior plausibility, and not only find it impossible to suspend disbelief while doing an impartial investigation, but also consider such investigations mostly a waste of time. For example, I consider claims of miracles to have such negligible prior plausibility as to make it impossible for me to investigate, say, an apparition of the Virgin Mary on a church wall except to discover what physical and psychological phenomena lead people to see something as a sign from a person who has been dead for over two thousand years, a person who allegedly was a virgin who gave birth to a god. I'm sorry. No amount of self-deception could ever lead me to say with a straight face that I am going to investigate these claims with an open mind. To do so, in my opinion, would be an example of being so open-minded that your brains fall out.

On the other hand, when investigating claims about cold fusion, quasicrystals, the health benefits of garlic, and many other topics that I can't automatically fit into a framework of knowledge, experience, and belief that would make me consider them as having near zero plausibility, I have no difficulty in suspending disbelief because I don't have any prior disbelief to suspend. With topics like acupuncture, on the other hand, I have no trouble being open-minded about it having physical consequences that can benefit the body, but I cannot pretend to take seriously the idea that needles are unblocking some sort of subtle "energy" in the process.

The fact is, though, that what has no prior plausibility for me may have some for you. If your worldview is filled with gods, spirits, subtle energies, and laws like the law of attraction or the law of similars--laws that no scientist has ever seen in operation--then you are going to consider many things as having prior plausibility that I cannot so consider. You may even consider many things I consider nearly impossible as having prior probability before you investigate them. Does this matter? Yes, in some cases. If I'm in charge of deciding what medical research to support, it does matter if I think homeopathy has near zero plausibility while you think it has a high degree of probability based on your many years of experience administering homeopathic products. I think it would be a waste of money to do studies on homeopathic remedies unless those studies are aimed at finding out what physical and psychological processes, if any, are at work in homeopathy that either provide or make people think there is a health benefit from water (or some other inert substance). Chasing after chimeras like "the memory of water" is an idea I find absurd on its face. I can't pretend this is possible because it would require me to suspend belief in what little knowledge I have of chemistry and biology and it would require the entire scientific community to abandon the consensus view on such things as Avogadro's number.

In other words, during the decades I've been investigating paranormal phenomena I have found some cases where I should be open-minded and not set out with debunking in mind. But I've also found many cases where, due to the prior implausibility of a claim, I have set out to do several things, one of which is to debunk the claim. For example, when I first heard of the claim that the Maya had predicted that the world would end in 2012, I thought of investigating, not debunking the claim. The Maya wouldn't have been the first and they won't be the last to make a doomsday prediction. But when I discovered that some people were claiming that the prediction was justified, I set out to debunk a claim that, for me, has zero plausibility. But that is not all I set out to do. I wanted to find out why anyone would believe the Maya would have such foreknowledge. I investigated the origin and history of the idea. I tried to understand why anyone would believe such a thing and what were the consequences of promoting or believing it. I could never pretend to be open-minded about the possibility that the Maya had some way of knowing when the world is going to end. Maybe they did predict the end of the world. So what? Anybody can predict anything. There is no problem with the prior plausibility of anyone making a claim they couldn't possibly know to be true. Maybe the Maya didn't predict the end of the world and those who were claiming they did misunderstood or misinterpreted something. That's plausible, even highly probable. Maybe somebody thought it would be fun to hoax people with a scary book or movie. That's plausible. In other words, I've found that many cases I investigate are mixed with elements that have zero plausibility, some plausibility, and some even have a good degree of probability. I think most paranormal investigators actually follow a similar process to mine, even those who do field investigations of ghost stories or miraculous statues.

Rather than being concerned about being a debunker or an open-minded investigator, I think I'm more concerned about jumping to conclusions about the motives of those who put forth ideas that have zero plausibility for me. I think it's important to give people the benefit of the doubt. Assume they're sincere until you have evidence that their belief is not based on some sort of evidence, even evidence you consider to be flimsy or highly questionable. Don't automatically think hoax when confronted with some weird claim you find totally implausible.

I have to remind myself that many of the claims I now consider to have near zero plausibility are claims that I could be open-minded about many years ago. So, when I am confronted by believers in ghosts, reiki, gods, ESP, or psychokinesis, for example, while I dismiss their beliefs, I don't dismiss their sincerity. I try to make an effort to explain why I find their claims unbelievable. Prior plausibility is not an inherent quality of claims. Prior plausibility is relative to the knowledge and experience of the believer or disbeliever.

Plausibility should be distinguished from possibility. I accept that the Bible stories taken literally by fundamentalists are possibly true. A male superbeing may have created humans out of dust or the first female out of the rib of the first man. It is possible that such stories are true in the sense that I can imagine them happening. But being conceivable is hardly a strong reason for believing anything. Being inconceivable, on the other hand, is a good reason for rejecting a claim. All the conspiracy theories in the world are conceivable, but that hardly makes them worth believing. Many Bible stories are implausible and some are improbable. But one claim I consider to have near zero plausibility is that some superbeing dictated those stories and that they are infallibly true. To give that claim any degree of plausibility I would have to give up reasoning altogether. If you want to go one step further and tell me that I should give up reasoning altogether and accept on faith that the Bible is the infallible word of some god, then we have so little in common that we may as well end the conversation right here.

I also accept that our current scientific knowledge could all be wrong, that there are laws of nature we don't know yet, and that our universe is intersected by multiple universes with different kinds of reality that we never or rarely perceive. These are possibilities. But these possibilities do not make ESP, homeopathy, or energy medicine any more plausible. So, to those who say science doesn't know everything and there are some things that do not follow the known laws of nature and can't be studied by contemporary science, I say those possibilities are implausible and ad hoc. Such a belief deludes you into thinking you are justified in clinging to beliefs that have near zero probability of being true.

To consider me closedminded because I won't accept the Bible as literally true or that all the laws of nature so far discovered should be disregarded to make room for homeopathy or reiki or psychokinesis is ludicrous. Following your lead, there is no end to the kinds of claims people might accept on faith or on the possibility that they might be true. It seems rather cavalier to accept science when it comes to treating your diabetes or building your automobile or power plant, but to reject it to make room for belief in an ancient god, a medical superstition, or a superhuman power.

Robert T. Carroll
14 March 2015

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