Who has the burden of proof for an extraordinary claim? Those who make the claim or those who challenge the claim?
At first, it is quite reasonable to give benefit of the doubt to a man who makes a claim, such as "I have a unicorn in my garden". After all, we have not searched his garden and have no proof that he has no unicorn, and it seems reasonable to take the word of a man who has not been shown to lie. So reasonably, we take his word at face value and allow that he may indeed have a unicorn in his garden.
When we visit his home, we are interested to see the unicorn, but when asked the man says he has a big garden and the unicorn isn't around the house at the moment, but it is there nonetheless. We're disappointed, of course, but we have no reason to disbelieve him.
Later the man makes a moral judgement about our behaviour, such as "it is wrong to eat wheat on a Wednesday", and when asked why this is so, the man replies that his unicorn told him so. We are irritated by such judgement, and surprised that not only this man claims to have a unicorn, but that it can speak also. So we ask to see this unicorn and ask why it is wrong to eat wheat on a Wednesday. We search the garden with the man and find no unicorn, but have no proof that none exists. When challenged to produce the unicorn, the man now claims that the unicorn is invisible, and is in all parts of the garden at the same time.
Now the man's claim is looking extraordinary for a number of reasons:
1. We have never seen a unicorn, and they are popularly believed to be fictional;
2. We have never heard an animal speak, and most people accept that only humans speak;
3. The man changed his story to suit his needs as we enquired further about his talking unicorn;
4. We have never had experience of invisible animals, and invisibility has never been demonstrated; and
5. The concept of something being everywhere at once appears contradictory.
So we conclude that on the balance of probability, the man's claim is bogus and put the onus on him to prove the existence of his unicorn.
We tell the man not to be so silly and judgemental about our wheat eating habits, and go about our business, trying to avoid aggravating someone who appears not to have a firm grasp on reality.
Then we find that the man is a teacher is our children's school and is telling them that his unicorn is real, that they should not eat wheat on Wednesdays and other rules about how to live their lives which he cannot substantiate, other than by claiming his invisible unicorn told him so, and the children must believe him.
So now we are angry that our children are being indoctrinated and ask the school to stop him. When we find that the school governors are unicorn believers too, that none of them can prove the existence of unicorns either, but fully support the indoctrination of our children, we are outraged. We speak to local politicians and media, and mount a campaign to stop this silliness. But then the government, many members of whom turn out to believe in pixies or leprechauns, equally without foundation, object that we are trying to restrict people's right to believe what they choose to believe, and criticise us for being arrogant and intolerant.
We hold our heads in our hands, wondering how these people can be so deluded, and how we can have our children taught math, English and science, without all this mumbo-jumbo. We realise that it is we who are in the minority, and the world deluded.
What should we do?