Definition
A thought experiment showing that the burden of proof lies with the person who first makes a claim. It is also widely used to call out a logical fallacy: pushing the work of disproof onto others by saying, "You can't prove me wrong, so I must be right."
A teapot floating in outer space?
Imagine someone tells you, "There's a tiny china teapot orbiting the sun on an elliptical path, somewhere between Earth and Mars."
It sounds like nonsense, so you reach for a telescope to check. But the person adds, "The teapot is too small to be seen by any telescope on Earth." Now there's no way to prove the teapot isn't there. You could search every corner of space, and they could just keep saying, "It's out there. You just can't see it."
The philosopher Bertrand Russell said it is perfectly reasonable for people not to believe a claim like this. Just because you can't prove something doesn't exist, that doesn't make the claim true. If you want people to believe something new, the person making the claim has to bring good evidence first.
The burden of proof is on the one making the claim
We run into the same trick in everyday chats and online debates. Someone says with total confidence, "There's an invisible fairy living in my closet. If you think I'm lying, prove there's no fairy!"
In logic, this is called shifting the burden of proof. It means insisting your far-fetched claim is right just because the other person can't produce proof that it's wrong. In a fair discussion, the person who says something exists has to go first and offer solid evidence, like photos or data.
What if we accepted every claim as true whenever nobody could prove it false? We'd have to believe in an invisible dragon on the far side of the moon, or a secret alien base deep underground. Every wild lie would count as fact. That would be absurd.
A bit more precisely: science and falsifiability
To be more precise, science holds that a theory only counts as a scientific claim if there is a way to show it could be wrong. The philosopher Karl Popper proposed this standard, and it is called "falsifiability."
Suppose a theory answers every criticism and every observation by saying, "That's because of an unknown force that human technology can't measure." That isn't science. It's no different from Russell's teapot, just a word game. The starting point of real knowledge is accepting that new experiments and observations could prove you wrong, and stepping up to be tested anyway.
So the next time someone spreads a baseless rumor or fake news and fires back, "Bring me proof that I'm wrong!", picture that teapot in space. The job of proving the claim belongs to the person who made the outlandish claim, not to you for doubting it.
Common misconceptions
If the other person can't produce evidence against my claim, my claim has been proven true.
Having no evidence against a claim only means it hasn't yet been shown to be wrong. It is not evidence that the claim is true. The burden of proof always rests with the person who made the claim.
Where you meet it
Someone claims aliens are secretly living on Earth, then demands that skeptics give solid proof that aliens don't exist.
A seller pushes a cure-all with no scientific testing, then boldly tells customers to prove for themselves that it doesn't work.
In one sentence
The responsibility for showing that a claim is true belongs to the person who made the claim, not to the person being asked to believe it.