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How do we know what’s true?

Dear Professor

People used to believe something was true because someone told them it was true, and that someone might have been a priest or a king or a parent or a book, but after a while people realised that wasn’t a very good way of telling what’s true from what’s not true. There were different priests, kings, parents and books all saying different things. They couldn’t all be right, so how could you tell which were right and which were wrong?

Then along came science, saying that the best way to know what was true was to test things in the real world, like by doing experiments. So if someone claims that wood doesn’t float you test that claim by getting some wood and placing it in water. If you do that, you’ll find that it does indeed float.

It doesn’t matter if a king or priest or parent or holy book says that wood does not float. You can do the experiment and see for yourself.

All science is based on that idea. Someone makes a claim and then someone goes off and tests that claim by doing experiments, and those experiments have to be repeatable. Whoever does the experiment and wherever it’s done, the result will be the same.

This is called the scientific method. It’s how we know what we know and it’s how we’ve been able to cure diseases and send astronauts to the moon.

But this isn’t just for science. It’s also used by historians to decide what happened in the past and it’s how we decide whether or not someone is guilty of a crime. Scientists, historians and police detectives collect and study evidence to decide what’s true and what’s false. They have to be careful though, as there are many traps you can fall into.

One is the trap of cherry picking. That means not looking at all the evidence but just looking at some of it. Picking the evidence you like just like picking cherries from a cherry tree.

This is a trap any of us can fall into, but it’s especially dangerous when you have an idea that you want to believe is true and so you just look at the evidence that supports your idea and you ignore or dismiss any evidence that doesn’t.

Suppose someone claims that all swans are white but they refuse to listen to people saying “but I saw a black swan once” and they refuse to look at any photos of black swans, or they dismiss those photos as fakes, despite there being thousands of photos of black swans.

Sometimes we want so badly to believe something is true that we close our eyes to any evidence that suggests it’s not true. We refuse to listen to people telling us things we don’t want to hear and we close our eyes to the evidence we don’t want to see.

Have you ever done that?

Most of us have, if we’re honest, but to be a scientist or to be like a scientist you have to be brave. You have to follow the evidence wherever it takes you. You have to keep your eyes open, you have to listen and you have to ask questions. If someone claims that something is true, ask them what evidence they have to support their claim.

Yours truly…