How do you know if something is true?

There are three very different reasons we believe things. Because it is easy to check. Because someone we trust told us. And because a lot of people say so. Only the first one is proof; the other two are clues.

A test you can use: what would make this false? If nothing could, it is not really a claim to believe - it is just a feeling wearing a lab coat.

Philosophers also separate two kinds of knowing. Knowing a fact (‘7 x 8 = 56’) and knowing how to do something (‘knowing how to ride’). You can have the first without a second, and the second without being able to explain it.

Try it

Play 'two sources'. Ask a grown-up about one news story or one family story. Then ask another person the same question. Write down what is the same in both answers, and what is different. Where they differ is where being careful matters most.

What do you think?

Do you think there is a right answer to this question?

Talk about it

Is it ever right to believe something with no proof?
If everyone believes it, is it true?
Can you be certain of anything at all?

Yes, but… — the arguments people actually make

Everyone believes it, so it must be true.
Everyone once believed the sun went round the Earth. That is a majority, not an argument — and majorities have been wrong at a steady rate for all of history.
You can't be certain of anything, so it doesn't matter what you believe.
Certainty and evidence are different things. You cannot be certain it is safe to cross the road, and you still look both ways.
My truth is different from yours.
About what you like, yes. About whether the door is locked, one of you is going to find out.

What other philosophers think

AristotleA statement is true when it says of what is, that it is, and of what is not, that it is not. Truth is a match between the words and the world.
René DescartesDoubt everything you can, all the way down, and build only on what survives. What does not survive was never knowledge.
Karl PopperThe mark of a real claim is that you can say what would show it to be false. If nothing could, it is not a claim about the world.

If you want to go further

Meditations on First Philosophy — René Descartes1641. Doubt taken to its limit, and one thing that survived.
Conjectures and Refutations — Karl PopperWhere falsifiability becomes a test you can actually apply.
Metaphysics — AristotleBook IV, the earliest careful account of what 'true' means.

Most of these were written for adults, which is why it is worth reading them with someone older.