How to provide a simple example to the requirement of falsifiability in the scientific method to a novice audience?

You have a bucket. You draw out a few red balls, and state that the "theory" is that the bucket contains only red balls. You emphasize that this is a falsifiable theory because it can be completely disproved by drawing out any other color of ball. For dramatic effect, you can proceed to draw out a blue ball.

If you have the opportunity to make it slightly more complicated, you can have another bucket, and state that your hypothesis about this bucket is that it contains 50% red and 50% blue balls. You start drawing out balls, and keep track of the count on a screen or whiteboard. The more balls you draw out, the more obvious it is that the ratio is something more like 90% red, 10% blue. This gives you the opportunity to talk about instances where the theory can't be disproven instantly by a single observation, but the burden of evidence nonetheless accumulates against it over time.

I think it would be great to start with a theory that sounds very scientific, but is unfalsifiable, and therefore useless. Then we modify the theory to include an element that is falisfiable, and the theory becomes much more useful.

For example, we have a new kind of medicine, and it is very good for some people, but when other people take the medicine it kills them. Naturally, we want to know who would be killed by the medicine, and who would be helped by it.

A scientist has a theory. He believes there is a gene that he calls the "Spottiswood gene". Anyone who has the proper form of the Spottiswood gene will be safe, they can take the medicine freely. But some people have a broken version of the Spottiswood gene, and they die when then they take the medicine. Unfortunately the scientist has no way of detecting the Spottiswood gene, so he can't tell you whether you have the gene or not.

Now this theory sounds very scientific and it's got lots of scientific words in it, but it isn't very useful. The scientist doesn't know how to detect the gene, so he can't tell you whether you are going to live or whether you are going to die. He can't tell you whether it is safe to take the medicine. If you take the pill and you survive, then the scientist will say that you had the working version of the gene. If you take the pill and you die, the scientist will say that you have the broken version of the gene. But he cannot say what will happen to you until after it has already happened, so his theory is useless. He can explain anything, but he can't make predictions in advance.

Now another scientist has a different theory. She thinks that the medicine is related to eye color. She thinks anyone with blue eyes will die if they take the medicine, and she thinks that anyone with brown eyes will be okay. She's not sure why this happens, but she plans to do more research and find out. Even if she doesn't do any more research, her theory is much more useful than than the first scientist's theory. If she's right, then blue-eyed people will know that they should avoid the medicine, and brown eyed people will know that they can take the medicine safely. She has made predictions. She predicts that no brown eyed person will die after taking the medicine, and she predicts that no blue eyed person will live.

Of course, the second scientist might be wrong. But the interesting thing is that if she's wrong, then we can prove that she's wrong. She predicted that no one with brown eyes will die after taking the medicine, so if lots of people with brown eyes die, then we will know that she's wrong.

If her theory is wrong, then we should be able to prove that it's wrong. And then if the results don't prove that she's wrong, we accept that she's probably right. That's called falsifiability.

But the first scientist doesn't have falsifiability. We know that even If he's wrong, we'll never be able to prove it - and that means we'll never know if he's wrong or right. More importantly, even he is right, his theory still wouldn't do anybody any good.

Personally, I'd start with the concept of predictive power, and then mention falsifiability as one way to notice if a statement makes no predictive assertions.

The point to make is that you're not claiming that non-falsifiable statements can't be true, it's just that they don't matter.

This depends on what kind of unfalsifiability you want. There are at least four kinds.

• unfalsifiable with current resources (Russell's teapot)
• unfalsifiable because of moving goalposts
• unfalsifiable because the terms are incoherent or undefined ("not even wrong")
• unfalsifiable in principle

No empirical claim is unfalsifiable in principle (i.e. without resource limitations, moving goalposts, or logical incoherency). Claims that involve violations of physical law come the closest, but require us to assume 100% confidence in the law itself. For a non-empirical claim to be unfalsifiable, empirical consequences of the claim have to be impossible, which ultimately requires you to eliminate them by definition. I think you’re trying to find an example of the fourth meaning when most people who talk about unfalsifiability are thinking about one of the others.

And if I make it interactive, it has some non-negligible chance to fail, especially if done with a broader audience.

You don't have to ask the whole audience. You can ask for a volunteer and ask people to raise their hands if they would be willing to volunteer. Then you pick a person who doesn't look like a nerd who already knows the problem.

Have a fallback plan: In case they do successfully answer the question you can ask them to explain why they ask the right questions.

Wikipedia lists the "all swans are white" as an example for a falsifiable statement, but it is not practical enough. To prove that all swans are white would require to observe all the swans in the world.

Something being falsifiable and something being universally possible to check are 2 different things.

In theory you could falsify that statement after checking only a single swan if it happens to be a black swan.

Conservation of energy is falsifiable. If you found some way of creating energy without taking it from elsewhere then you would falsify it. However it isn't practical to check every cubic meter of space in the universe to check if it applies everywhere.

there's also the old Invisible Dragon example from Sagan

“A fire-breathing dragon lives in my garage.”

Suppose … I seriously make such an assertion to you. Surely you’d want to check it out, see for yourself….

“Show me,” you say. I lead you to my garage. You look inside and see a ladder, empty paint cans, an old tricycle—but no dragon.

“Where’s the dragon?” you ask.

“Oh, she’s right here,” I reply, waving vaguely. “I neglected to mention that she’s an invisible dragon.”

You propose spreading flour on the floor of the garage to capture the dragon’s footprints.

“Good idea,” I say, “but this dragon floats in the air.”

Then you’ll use an infrared sensor to detect the invisible fire.

“Good idea, but the invisible fire is also heatless.”

You’ll spray-paint the dragon and make her visible.

“Good idea, except she’s an incorporeal dragon and the paint won’t stick.”

And so on. I counter every physical test you propose with a special explanation of why it won’t work.

Now, what’s the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all? If there’s no way to disprove my contention, no conceivable experiment that would count against it, what does it mean to say that my dragon exists? Your inability to invalidate my hypothesis is not at all the same thing as proving it is true. Claims that cannot be tested, assertions immune to disproof are veridically worthless, whatever value they may have in inspiring us or in exciting our sense of wonder. What I’m asking you do comes down to believing, in the absence of evidence, on my say-so.

You are out to get me. If you're mean to me, that's direct evidence. If you're nice, that's also evidence that you are trying to trick me into trusting you. If you ignore me, well that's the worst of all because it proves that you are trying to divert my attention from you so you can plan your attacks on me in secret.

Depending on the time scale of the experiment, you could run a rain-dance trial. (thinking primary school kids)

a rain dance is where you do a special dance to make it rain. spend 5 minutes each day doing the dance, (or not) and evaluating if it rained yesterday, keep a graph of rain+dancing. Run trials for as many weeks as you like.

Some other rules for the 2-4-6 game, so you can keep going if they get the first one:

• The set has an even number in it

• All three are 1-digit numbers

• All three must be numbers

• Whatever the numbers are, you alternate between replying 'yes' and 'no' (helps to be writing them down in this case)

Try to have more than one hypothesis under consideration at every time, and choose guesses which distinguish them.

I don't see how 2-4-6 is about falsifiability, so I may be misunderstanding your request. In the sequences, it was described as an example of positive bias. Clearly at every step, if the answer was "no" when you expected it to be "yes" the theory would be falsified.

https://en.wikipedia.org/wiki/Russell%27s_teapot

I like the story of Feynman and Tukey counting, although it mixes up falsification with behaviorism. html pdf

Since then I found a partially relevant, but very simple and effective "puzzle".

There are four cards in front of you on the desk. It is known, that every card has a numerical digit on one side, and a letter from the English alphabet on the other side.

You have to verify the theory that "if one side of the card has a vowel, the other side has an even number", and you are only allowed to flip two cards.

The cards in front of you are:

A T 7 2

Which cards will you flip?

(I wrote partially relevant because this is not an example for an unfalsifiable theory. The theory is falsifiable and the puzzle is solvable, the main point is that most people would pick the wrong answer because they will not try to falsify the theory)

Go through a Venn diagram explanation of Bayes's Theorem. Not necessarily the formula, but just a graphical representation of updating on evidence. Draw attention to the distribution of probability of H between E and not-E. Point out that if the probability of H doesn't go down upon the discovery of not E, it can't possibly go up upon the discovery of E.

This has the advantage of showing the requirement of falsifiability to be an extreme case of a more powerful general principle.

This could be supplemental to some of the great suggestions by your other commenters.

Take Howard Gardner theory of multiple intelligences. The world is fair. Some people have musical–rhythmic intelligence while other people have logical–mathematical intelligence.

Gardner theory has intuitive merit. But if you start to think about falsifiabilition you come to the question of whether the multiple intelligences really are different or whether they correlate positively with each other.

Maybe the Placebo effect, all medications have affect the patient (even if it does nothing) so you can not prove a medicament does not work without a control group using Placebo to make the claim falsifiable.

Does the flying spaghetti monster work?

Or the example in http://lesswrong.com/lw/ip/fake_explanations/?

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