In the last few hours I have twice had the old maxim “Extraordinary claims require extraordinary evidence” quoted to me. I have dealt with this principle before (see here), but that was a couple years ago, so it would seem like a good time to return to the topic.
In the spirit of irenicism, let me begin with some kind words about the principle. It is, first of all, an apt description of how people reason. That is, when people find a particular claim extraordinary then they are more resistant to accepting it, and it takes that much more in terms of evidence, time, and winsome presentation to persuade them to rethink their position. In that case the principle is simply a matter of apt psychological description.
However, the principle is presented not merely as a summary for how people do reason, but also how they ought to reason. That is, if a claim is extraordinary then you really ought to seek a high grade of evidence for it.
Let’s say that Smith meets two of his new neighbors at the annual block party. The first neighbor says:
“Hi, I’m Bob. I just moved from Las Vegas.”
Smith will likely consider Bob’s testimony adequate to believe he did indeed just move from Las Vegas. After all, there’s nothing particularly unusual about the claim. But then Smith turns to meet the next neighbor and he hears this:
“Hi, I’m Dave. I just moved from the planet Saturn.”
Chances are that Smith will not accept Dave’s testimony of his origins without further supporting evidence. This despite the fact that he did accept Bob’s testimony. So what’s the difference? Is Smith being unfair? Hardly. The obvious difference is that Dave’s claim is extraordinary while Bob’s is quite mundane. This is the principle in action.
To sum up, the principle is laudable both as a psychological description of how people typically reason and as a normative rough and ready principle for how they ought to reason. So wherein lies the disagreement?
The problem arises with the fact that those who invoke the principle typically fail to recognize that it is always applied relative to systems of belief and experience. And since systems of belief and experience differ, folks will differ as to what they find extraordinary.
Consider as an example the different way that theists and atheists process the claim that Jesus was resurrected. Atheists often invoke the principle to dismiss the hypothesis that Jesus rose from the dead. And granted, if one starts as an atheist then the belief that Jesus rose from the dead will appear extraordinary. But if one is a theist then one assesses a resurrection claim relative to theism. And as a result the claim becomes a viable possibility.
Or consider the origin of the universe. It seems to me that the claim that the universe sprang into existence uncaused out of nothing must surely be one of the craziest, most extraordinary claims one could make. But at least some atheists consider the claim a live option while dismissing that which I take to be perfectly reasonable, namely, that God created the universe.
Or consider cosmic fine-tuning. The theist finds this fitting perfectly well with the divine intention to create a universe specified to the creation of life. Meanwhile many an atheist will opt instead to explain this apparent fine-tuning by invoking trillions of universes, only one of which happens to be life permitting.
Time and again I’ve heard atheists wield the principle in a way where it presents the set of beliefs they hold as conforming perfectly to that which is “ordinary” while everything the theist might say is relegated to the “extraordinary”. Of course this also works the opposite way with some theists wielding the principle to ensure that they come out perfectly reasonable while the atheists are paragons of irrationality. But once we recognize that judgements of that which is ordinary and extraordinary are always made relative to systems of belief, we can stop using the principle as a final declaration to marginalize other people, and use it instead as an invitation to open dialogue as each side challenges their interlocutor to redraw the line between the ordinary and extraordinary.