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26 October 2006

Newcomb's Paradox: Reversing Causality?

Justin Grimmer

Newcomb’s paradox is a classic problem in philosophy and also an entertaining puzzle to consider. Here is one version of the paradox. Suppose you are presented with two boxes, A and B. You are allowed to take just box A, just box B, or both A and B. There will always be $1000 in box A, and there will either be $0 or $1,000,000 in box B.

A ‘predictor’ determines the contents of box B before you have arrived, using the following plan. If the predictor believes you will pick both box A and B, then she places nothing in box B, but if she believes that you will only take box B, then she places the $1,000,000 in box B.

What makes this predictor special is her amazing accuracy. In the previous billion plays of the game she has never been wrong.

So, you have the two boxes in front of you, what should you do? Keep in mind, the predictor has already made her decision when you arrive at the boxes, so by our normal rules of causality (events in the future cannot cause past events), our actions cannot change what the predictor has decided.

Posted by Justin Grimmer at 12:00 PM