If you were wrong, how would you know?

A common sense guide to falsifiability

“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. There have been innumerable stories of dragons over the centuries, but no real evidence. What an opportunity!

“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 to do comes down to believing, in the absence of evidence, on my say-so.

— Carl Sagan

What makes science science? What distinguishes science from other fields of study, such as art, music, poetry, literature, philosophy, religion, theology, mythology, etc?

The (mainstream) answer, provided by the philosopher Karl Popper in the early twentieth century, and accepted by most scientists today, is the criterion of falsifiability. The definition of “falsify” is “prove (a statement or theory) to be false.” For an idea to be falsifiable, it must be possible, in principle, for the idea to be proven wrong. For example, the statement “all swans are white” is a proper scientific hypothesis, because observing a single swan that is any color other than white would prove the hypothesis to be false. And it just so happens to be a falsified hypothesis, because black swans have been discovered.

A theory which makes bold, testable predictions so that it is highly open and vulnerable to falsification, and yet it resists being falsified for long periods of time, is a strong theory. A theory which is either unwilling or unable to specify what would constitute a disproof so that it is compatible with every possible version of reality, is not scientific. In fact, it’s not even wrong. It is what scientists and philosophers would call “unfalsifiable”.

What are some examples of unfalsifiable ideas?

1. The Multiverse Hypothesis. The idea that our universe is one out of many.

2. The Simulation Hypothesis. The idea, popular among sci-fi dorks, that our universe is a digital simulation running on some alien’s computer.

3. The God Hypothesis. The idea, popular among religious people, that the universe was created by a supernatural agent.¹

All of these ideas are fun and fascinating thought experiments, but since there is no conceivable way of disproving them, they are unfalsifiable and therefore non-scientific. Those ideas could all be true for all we know, but we can never really be confident that they are, because they make no testable predictions.

How do you know if your idea is falsifiable? Ask yourself: If I was wrong, how would I know? This simple question (which will dramatically improve the quality of your thinking, if you incorporate it into your way of thinking) is the foundation of science. There’s two steps to this process:

1. You acknowledge the possibility that your idea could be disproven.

2. You specify what exactly would constitute a disproof.

Does your idea make any predictions? What would be a contradiction? What would be a counterexample? What would be a disproof? Fundamentally, how would the world be different if your idea was true? If you are not able to answer any of these questions, your idea is non-scientific.

What are some examples of falsifiable ideas?

1. All swans are white. As mentioned above, you could falsify this idea by finding a single swan that is not white. This idea has in fact been falsified.

2. Sphericity of the Earth. You could falsify this idea by taking a photograph of the Earth from space, which shows the Earth to be a cube, a pyramid, or some other shape.

3. The theory of evolution. The idea that livings things evolve over time. This idea could be falsified if the fossil record showed no changes over time, or if anachronistic fossils were discovered.

4. The germ theory of disease. The idea that specific diseases are caused by specific germs. For example, we know that the disease COVID-19 is caused by the virus SARS-CoV-2. This idea could be falsified if you could show that there is no causal relationship between the manifestation of disease symptoms (coughing, etc) and the presence of the virus in the body.

1. Note, however, that some have argued that this hypothesis is falsifiable and has actually been falsified.