What the slogan means and how it gets misused
You can’t travel far on the internet without hearing the slogan: Extraordinary claims require extraordinary evidence. It’s often announced as if merely repeating it settles the matter, without any explanation of what counts as extraordinary.
The saying was popularized in Carl Sagan’s 1979 book Broca’s Brain in reference to paranormal claims, but it’s since become a standard reply to any idea people find hard to believe.
It’s also found its way into scientific writing. It’s been called “a fundamental principle of scientific skepticism” and claimed to be “at the heart of the scientific method, and a model for critical thinking, rational thought and skepticism everywhere.”
If the slogan is a fundamental maxim of skepticism, it deserves the same scrutiny skeptics give any other claim.
What makes a claim extraordinary?
What type of evidence is extraordinary?
Tracing the origin of the slogan
To answer these questions, we can trace the origin of the idea to the philosopher David Hume’s essay, Of Miracles (1748), which is part of his classic work, An Inquiry Concerning Human Understanding.
In his essay Hume gives us an accurate description of what extraordinary means. Hume thinks evidence is derived from our experience of the world, and the laws of nature are established based on repetitions of those experiences.
Hume argued that a miracle requires extraordinary proof or evidence because by definition, a miracle was “a violation of the laws of nature” and those laws of nature were established by countless experiences of them happening.
To believe a miracle has occurred was “a contest of two opposite experiences”. To witness a miracle is to have a single experience of the laws of nature being violated, but against this experience we have the many experiences that caused us to consider it a law of nature.
Hume gave as one example of a miracle, a piece of lead held in the air which remains suspended when released. If we’ve observed a thousand times that lead will fall to the ground when dropped, to believe a miracle has occurred requires a thousand and one observations of it remaining suspended in the air.
Whatever you think about the success of Hume’s empirical argument, his intention about the meaning of extraordinary is clear. Extraordinary means large amounts of evidence. For a claim to be extraordinary means it already has large amounts of evidence to the contrary.
There isn’t some different type of evidence we need, or some different type of claim we’re assessing, extraordinary refers to the quantity of evidence we already have. An extraordinary claim is one for which we already have an extraordinarily large amount of evidence it is false.
What extraordinary refers to
The confusion comes from calling the claim extraordinary, when extraordinary refers to the evidence. There aren’t different types of claims, there are different amounts of evidence for claims. Sometimes we have a lot of background evidence for a claim so we can easily say if it’s true or false. Sometimes we don’t have much evidence for a claim, and we might withhold judgement on its truth and remain agnostic.
But the slogan is often used more loosely to dismiss anything someone personally finds hard to believe.
It’s often used to reject anything that sounds like pseudoscience because it’s a claim for which we currently have no scientific evidence. But that’s a different standard than the one Hume described and it risks closing off open minded enquiry.
There’s a crucial difference between phenomena which are outside the laws of nature and those which are outside current scientific understanding. Einstein once called quantum entanglement spooky action at a distance. The Big Bang is said to have been coined as a joke about what is now a well-established scientific theory.
More importantly, focusing on phenomena that aren’t well understood is how scientific progress is made. As Thomas Kuhn said in his classic work, The Structure of Scientific Revolutions, “discovery commences with the awareness of anomaly”.
This matters because the slogan can be used to rule out ideas which don’t fit a materialist viewpoint.
What counts as ordinary?
There is often the suggestion that natural claims are ordinary and reasonable, and all non-scientific claims are extraordinary and shouldn’t be believed. But this is only a colourful way of speaking, but it can make naturalism seem like the default and so opposing views are extraordinary and unbelievable.
Personal incredulity isn’t a good reason to reject a claim. Philosophers have a phrase for responding to claims with expressions of personal disbelief as consisting only of an “incredulous stare”. It avoids the argument rather than answering it.
Once we see what extraordinary refers to we find there are only plain old claims, the plain old evidence we have for those claims, and the ordinary work involved to assess their truth.
Treating some claims as extraordinary can mislead people how to think critically. It gives the impression some claims are given different treatment than others, whereas it’s really the evidence we should focus on.
The substance of the slogan reduces to the saying: Claims require evidence in proportion to how much we already have against them. That’s a reasonable principle. The problem is when it’s used to avoid looking at the evidence, calling a claim extraordinary because it is unusual or doesn’t fit our current beliefs, rather than because we have a lot of evidence against it.
