ECREE: extraordinary claims require extraordinary evidence?

"Extraordinary claims require extraordinary evidence!" Ever since Carl Sagan said it, this aphorism (often abbreviated to the acronym ECREE) has been used to critique claims of miracles or the paranormal. It sounds neat and logical, and it can help you feel you have the 'high logical ground', but is it true?

Before we decide, we need to better understand what it is saying.

what are extraordinary claims?

We need to define this, otherwise anyone could simply say they find a claim extraordinary, or ordinary, and so avoid further argument. But it isn't so easy to find a consistent definition. Here are a few suggested definitions of 'extraordinary':

• unprecedented - but many unprecedented things are not extraordinary (e.g. my enjoying eating yoghurt);
• beyond what is commonly believed - but belief in both God and miracles are quite common, yet some would want to call these extraordinary claims;
• difficult to believe - but what is difficult for one person to believe is easy for another, so who gets to define this, and how?
• One blogger defied it as "scientifically contentious" presumably meaning not explicable by science. This surely gets a little closer, but still fails to cover matters which are not resolvable by science, for example, historical questions (e.g. did Jesus exist?) or personal questions (e.g. Cinderella asking "Can the handsome prince really love a poor girl like me?").

The common theme in all these attempts at definition is antecedent probability - how likely is it that events like this could occur? For example, if we are considering a particular claim that a miraculous healing has occurred, how probable are miraculous healings in general?

But of course this pretty much begs the question, because we will all have different answers. There may not be a scientific explanation for miraculous healings, but many people claim to have experienced healing. So it is likely that people will choose a definition of "extraordinary claims" according to their worldview - what I require to convince me.

what is extraordinary evidence?

This is even harder to define and justify. What makes evidence 'extraordinary'? Surely what we require of evidence is that it be convincing. For example, if I claim that I have won ten consecutive lotteries, the odds are strongly against such an event and it is hard to explain it, so it might be classed as extraordinary. But the evidence required to support my claim is nothing extraordinary - merely ten winning lottery tickets, commonplace in themselves but unusual in that I have all ten.

a new defintion

So we may need to re-phrase the aphorism into something less memorable but more accurate: "If I think something is unlikely to occur, I will require reliable evidence before I believe it."

That re-statement is eminently sensible, and few would disagree with it, but it is so subjective that it cannot easily be used to settle arguments. Let's examine how it might work.

is it reasonable to believe the resurrection of jesus?

Suppose we accept, what the historians generally agree on, that Jesus' followers believed in the resurrection right from the beginning, because his body had disappeared and they had had visions of him. How will an atheist and a christian apply ECREE?

The atheist will claim that science had proven that dead men don't return to life after several days, so ECREE demands an extremely high degree of evidence, which isn't available. The christian will agree that in the natural course of events, dead men don't rise, but this isn't a natural event. The claim is that God raised him, and because of the unique claims Jesus made, that is not an extraordinary claim, and historical evidence is quite satisfactory. The atheist will argue that there's no evidence for God, but the christian will again disagree, perhaps pointing to one of the philosophical "proofs".

And the argument will go in circles, because each party has a different assessment of the antecedent probability that Jesus could possibly have been raised to life by God, and hence a different assessment of the evidence required.

so what of ECREE?

Doubtless it will continue to be invoked, and doubtless believers of all types will continue to point out its weaknesses and subjectivity. It will therefore be useful for those who prefer a slogan to an argument, but those who want to have logical discussion will use a modified form of it, which comes back to the philosophical given of: "All propositions require evidence of their truth."