How Much Evidence Does a Strange Claim Need?

Hume, Bayes, and being honest about where we start

In the last two series, you asked me hard questions. Is a cardiac arrest the same as brain death? Aren’t these reports just subjective? Can they be repeated? What would count as proof? Some of you asked politely and some of you didn’t, and all of those questions were fair. I owe you straight answers, and I intend to hold my own claims to the standard I’d hold anyone else’s.

So I’ll start where the argument really starts. How much evidence does a strange claim need?

Hume's rule

In 1748, David Hume wrote his essay on miracles. His rule was simple. A wise person fits belief to the evidence. When someone reports something that goes against all of our experience, you weigh two things: how likely is it that the report is wrong, and how likely is it that the event happened? You believe the report only if its being false would be the bigger miracle.

I like that rule. It doesn’t ban belief in strange things. It asks you to do the sums. Most people who quote Hume have skipped the sums.

Sagan's slogan

Carl Sagan made the idea famous with “extraordinary claims require extraordinary evidence". He wasn’t the first. The sociologist Marcello Truzzi said something similar in the 1970s, and the mathematician Laplace said it in the early 1800s.

I agree with the slogan, but it is quoted far more often than it is examined. It leaves two questions wide open. Who decides what counts as extraordinary? And how much extra is enough? Too often “extraordinary” just means “I don’t like it", and “enough” means “more than you’ll ever produce". That isn’t a standard; it’s a veto.

Bayes makes it concrete

Thomas Bayes gave us a way to answer both questions without hand waving. How far a piece of evidence should move you depends on two things: where you started, and how likely that evidence would be if the claim were false.

If you start with a very low chance, you need evidence that would be very unlikely to turn up if you were wrong. That is a high bar, but it is still a bar, and bars can be cleared. The one thing Bayes will not let you do is set your starting point at exactly zero. Do that, and no evidence on earth can move you.

Statisticians call this Cromwell’s rule, after Oliver Cromwell’s plea to the Church of Scotland: “I beseech you, in the bowels of Christ, think it possible you may be mistaken.”

Bars have been cleared before. In 1803, the French scientist Jean-Baptiste Biot investigated a shower of stones that fell near the town of L’Aigle. The scientific establishment had dismissed the idea of rocks falling from the sky as peasant superstition. Biot interviewed witnesses, mapped where the stones landed, and showed the pattern fitted a single fall. Meteorites became accepted science.

In 1984, an Australian doctor, Barry Marshall, drank a culture of a bacterium to show it could cause serious stomach disease (gastritis). Before, doctors had believed stress and acid were to blame. Marshall and Robin Warren won the Nobel Prize in 2005.

I’m not telling those stories to say that the establishment is always wrong. It usually isn’t, and being laughed at proves nothing. Plenty of people have been laughed at because they were wrong. I tell them because both were won with evidence the sceptics couldn’t wave away: physical specimens, careful mapping, a test anyone could check. That is the standard I want, and it’s the one I’ll be held to.

Where Hume went too far

Hume leaned on the claim that our experience is “uniform” against miracles. The philosopher John Earman argued in 2000 that this move quietly assumes its own conclusion. If you decide in advance that experience is uniform against something, no testimony could ever be strong enough, however many witnesses and however careful they are. That isn’t weighing evidence. That’s a closed door with a philosophical lock on it.

I see the same move today, usually from people who’d be offended to be compared with a believer. They have a conclusion and are working backwards.

Where I'm starting from on evidence

Your starting point matters more than most people admit, and it’s rarely stated. A committed materialist begins with a near zero chance that consciousness could work any way other than as a product of the brain. A committed believer begins with a high one.

When two people start that far apart, the same evidence looks completely different to each of them, and they can spend decades talking past each other while each thinks the other is being stubborn.

So here is mine. I think it is more likely than not that the brain does more than generate consciousness, and that it may shape or filter it. I hold that loosely. I also know it comes partly from my own experience, and my own experience is not evidence anyone else is obliged to accept. If you start somewhere else, I’m not insulted. I’d just like you to tell me where.

Over the next five posts I’ll set out the standard I would hold myself to, and what would change my mind. Next up: does a subjective report count as data at all?

Kathy Postelle Rixon is Chair of The Philosophical Society: Oxford, an independent researcher in consciousness studies, and a shamanic practitioner of more than forty years, working in Q’ero, Norse and Celtic traditions. You can contact her at kathy@magicinharmony.com or visit her website at www.magicinharmony.com.