Extraordinary claims require extraordinary evidence

The title quote is a popular one with skeptics, and is often attributed to Carl Sagan in his Cosmos series, though Wikipedia reckons that Marcello Truzzi and Pierre-Simon Laplace both said something pretty similar first. I think I prefer another assertion along the same lines, known as Hume‘s maxim, which states:

That no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavors to establish.

This, I think, expresses what Sagan and the others were getting at, perhaps a little less succinctly, but also more clearly and unambiguously. It’s closely related to Occam’s Razor, or the law of parsimony, but it deserves some elaboration as to its exact meaning. It’s not really fair if “extraordinary evidence” is allowed to become a vague and unattainable moving goal-post for any phenomenon not already established.

If you met me at a party, and I introduced myself as Greg, you’d have little reason to doubt me on this point (assuming you don’t already know that that’s not my name) and probably assume it to be true without a moment’s hesitation. Why would I be lying about my own name, to someone I’m meeting in some casual social situation? It’s hardly an extraordinary claim, so extraordinary evidence is not really required – my word is probably enough to convince you.

If I were to further claim to have made a considerable fortune by hiring myself out as a private contractor to service the sexual needs of Natalie Portman, I’d probably lose a smidgen of credibility. The claim is, after all, on the incredible side, and the evidence I’ve provided is rather lacking, so you’d be reasonable to assume that I’m probably attempting to live out some personal lonely fantasy.

Looking at it in Hume’s terms, it’s unlikely to be more than a very minor miracle for me to give you my real name. I could be lying for some reason, but who does that? It’s not impossible, but marginally more miraculous, so you’re probably safe assuming you do know my name.

However, my earning any money at all as a man-whore to today’s Hollywood starlets would make raising Lazarus from the dead look about as impressive as stealing a kid’s nose. And all you’ve got is my testimony, which ain’t much. It might be odd that I’d tell such a brazen lie – most people aren’t mentally ill, and don’t just make up obvious nonsense to try and impress random strangers – but any explanation is more plausible than that I’m telling the truth. The falsehood of my testimony is hardly miraculous at all in comparison, so there’s probably not much point asking for advice on getting into my line of work.

We’ve all seen people lie for personal gain, or be wrong but sincere in their efforts to convince us of something they believe. I can’t even be bothered to find any links or references to corroborate it right now – that’s how obvious it is that people might easily claim to believe all sorts of untrue or incorrect things, no matter where they fall on the scale of honesty. If someone says, for instance, that Elvis beamed down into their bedroom last night from his new home planet, then there’s nothing very miraculous in suggesting that they’re either mistaken or lying.

If, however, the King then attends a major press conference announcing his new comeback tour, dazzles millions with a series of concerts around the world, and provides several independent scientific agencies with samples of the technology he uses for his interstellar travel, then we might start to be convinced. There would come a point where the “everyone’s just making it up and/or experiencing a mass hallucination” idea falls apart.

And the principle is often applied in actual, non-Elvis-based science, too. When Einstein first published his ideas about the curvature of space-time, gravitational time dilation, and other aspects of general relativity, it was pretty extraordinary and/or miraculous stuff. The evidence initially in its favour couldn’t have been called extraordinary, and for it to have been false didn’t seem to require anything more miraculous than a German scientist with some wacky ideas.

But then the evidence started coming in to support it, observations which Newtonian physics alone was unable to explain. When light was observed to bend around a massive object like the sun in just the way he predicted, in a way inconsistent with classical predictions, the falsehood of Einstein’s testimony became more and more unlikely. So these ideas, which started off sounding improbable and being rejected by most scientists, are now widely accepted. The extraordinary evidence has been provided, the miracle confirmed, and science has moved forward.

It’s a shame the ufologists, psychics, homeopaths, and so many other paranormal experts haven’t had similar luck.

The controversy around this principle comes, I guess, in making a judgment call on just how miraculous something is, and how reliable the testimony in comparison. This is bound to be at least partially a subjective measurement, but we can try and minimise how arbitrary and personal some of the decisions are. We should want to avoid setting our bar for “extraordinary evidence” so low that we become gullible and easily taken advantage of, and to do that well it’s worth understanding some of the ways that people can get things wrong. This is the kind of thing I’m planning to keep listing under ‘Wrongness’, near the bottom of the Skeptictionary list on the right there: the various kinds of faulty reasoning and misleading experiences that we can easily fall prey to, thanks to the unreliable nature of our squishy brains what do all our thinking.

If someone wakes up in the morning with a vivid memory of lying paralysed in their bed a few hours ago while a demon hovered over them, they may find this to be sufficient evidence that they were visited by something horrible from the pits of Hell. If they later read about sleep paralysis and find out about our understanding of the brain activity that can bring about these sensations, and the tests that have been done to examine what causes it and what might stop it, will they feel reassured?

Some people who go through this aren’t reassured, whatever they learn. I’ve never had it happen to me, but I understand it’s a pretty scary experience, which I can imagine feeling very real and leaving a lasting effect. But let’s take a less contentious example. I doubt many children have ever been seriously impressed by that trick where it looks like you’re detatching your thumb, but if they were, they were probably successfully disillusioned after being shown how it was done. Once you’ve seen how it can easily be faked, you assume that’s what was happening. It doesn’t make much sense to keep believing that Dad is just like Lego.

The same principles are at play in both cases. For a claim as extraordinary as being visited and imprisoned in your own body by minions of Satan, is “I remember feeling it happen, at around 4am, right after I remember running naked to catch a school bus” sufficiently extraordinary evidence? Is there anything particularly miraculous about the falsehood of the testimony – the idea that it might not have really happened the way you remember, given the other freaky shit we know your brain sometimes comes up with when you’re half-asleep?

I’ll try and go into some of the examples I’ve mentioned here in more detail later. For now, just take away the fact that, if you’re going to make a radical claim that undercuts vast swathes of established scientific understanding or invokes new and unimagined forces beyond the ken of man, you’re going to have to make a convincing case that there’s definitely something new going on the way you describe it, before we stop laughing at you and decide to completely re-evaluate our understanding of the world. It’s just basic skepticism.