ELI5: Explain Like I'm 5

newcomb's problem

Newcomb's problem is like a game or a puzzle where you have to guess what someone else is thinking.

Pretend you're playing this game with a person called "the Predictor." The Predictor has two transparent boxes in front of them: Box A and Box B.

Box A is made of glass, and we can see that it has $1,000 inside it. Box B is also made of glass, but we don't know what's inside it.

The Predictor tells you this: "I already predicted what you would choose to do before you even walked in here. If I predicted that you would only choose Box B, then Box B has $1 million inside it. If I predicted that you would choose both boxes, then Box B only has $1 inside it. Now, choose whether you want to take just Box B or both Box A and Box B."

This is where the puzzle comes in. If you choose to only take Box B, you either get $1 million or just $1, depending on what the Predictor predicted. But if you choose to take both Box A and Box B, you're guaranteed to get the $1,000 in Box A, but you may end up with only $1 in Box B.

So, what do you do? It seems like the safe choice would be to take both boxes and get the guaranteed $1,000, right?

But some people argue that if the Predictor is really good at predicting our choices, then they've already seen that we would choose both boxes, and that means they only put $1 in Box B. So, according to this line of thinking, we should choose to only take Box B, because there's a better chance that the Predictor saw that we would do that and put $1 million in Box B.

This is a tricky puzzle because it asks us to think about what we think someone else might be thinking, and also whether we believe in fate or free will. There's no one right answer, and people have been debating this puzzle for a long time.