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My son says you can’t prove fairies don’t exist. Why is that a weakness, not a strength?

Thamarai Raman Fellow
· 5 replies
Scientific methodAnalyticKarl Popper#burden-of-proof#demarcation#falsifiability#philosophy-for-children
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My nine-year-old has discovered the perfect argument. Fairies live in our garden, he says. When I ask how he knows, he says: “You can't prove they don't.” He is very pleased with this and uses it on his sister for everything from fairies to whose turn it is on the swing.

I know there is something wrong with it, but when I try to explain I end up saying “because I said so”, which is not a good example to set.

I did accountancy, not philosophy. In accounting, if a figure can't be checked, it doesn't go in the books. But that feels like a rule we made up, not a reason. Why exactly is “you can't prove it false” a weakness in a claim rather than a point in its favour? And is there a way to explain this to a child that doesn't just sound like adults winning by being adults?

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5 answers

Accepted answer

  1. Emeka Anyaoha

    ✓ Accepted answer Fellow

    The philosopher behind your instinct is Karl Popper. His idea: a claim tells you something about the world only by ruling something out. “All swans are white” is informative because a black swan would show it false. The more a claim forbids, the more it says. A claim compatible with every possible observation forbids nothing, so it says nothing about what you'll find, however appealing it sounds.

    So “you can't prove fairies don't exist” is true, but it's true of anything you might make up: invisible dragons, a china teapot too small for any telescope circling the sun, a second sister nobody has met. Being unrefuted is cheap. It can't count in favour of a claim if every invented claim gets it free.

    For a nine-year-old, try this: “What would our garden look like if there were no fairies?” If he says “exactly the same”, then the fairy claim isn't telling us anything about the garden. If he says “the flowers would be bent differently”, wonderful, now you can go and look together.

    One honest caveat: philosophers no longer think Popper's test draws a perfectly sharp line between science and everything else. But this everyday use of it, that a claim must risk something to be worth believing, has held up very well.

Other answers

  1. Arvind Poojary

    Fellow

    I hear the grown-up version of this in villages every month. A healer says his charm cures fever. If the child recovers, the charm worked. If not, the family lacked faith. The claim cannot lose, and that's precisely what's wrong with it: whatever happens, it has the same answer ready. A claim that survives every possible outcome isn't strong. It's empty.

    Helpful · 3
  2. Hsieh Wei-ting

    Fellow

    Game designer here. A rule that never triggers under any circumstances does nothing in the game; players eventually just ignore it. That's a nice way to say Emeka's point to kids, who all play games.

    Helpful · 1
  3. Thamarai Raman

    Fellow

    I tried Emeka's question at dinner. He thought for a while and said: “The garden would look the same, because fairies hide when you look.” His sister said that was cheating. I think she has a future in philosophy.

    Helpful · 2
    1. Lars Eriksen

      Fellow

      Your son has just reinvented a move physicists also make: protect the theory with an extra assumption. Astronomers once explained an odd orbit by an unseen planet, and found Neptune. What made that legitimate is that the extra assumption could itself be checked: point the telescope there. So the next question for him is: “Is there any way to catch a fairy hiding?” If there isn't, his sister is right.

      Helpful · 3