nshepperd comments on Newcomb's Problem and Regret of Rationality - Less Wrong
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Suppose my decision algorithm for the "both boxes are transparent" case is to take only box B if and only if it is empty, and to take both boxes if and only if box B has a million dollars in it. How does Omega respond? No matter how it handles box B, it's implied prediction will be wrong.
Perhaps just as slippery, what if my algorithm is to take only box B if and only if it contains a million dollars, and to take both boxes if and only if box B is empty? In this case, anything Omega predicts will be accurate, so what prediction does it make?
Come to think of it, I could implement the second algorithm (and maybe the first) if a million dollars weighs enough compared to the boxes. Suppose my decision algorithm outputs: "Grab box B and test it's weight, and maybe shake it a bit. If it clearly has a million dollars in it, take only box B. Otherwise, take both boxes." If that's my algorithm, then I don't think the problem actually tells us what Omega predicts, and thus what outcome I'm getting.
The naive presentation of the transparent problem is circular, and for that reason ill defined (what you do depends on what's in the boxes depends on omega's prediction depends on what you do...). A plausible version of the transparent newcomb's problem involves Omega:
Predicting what you'd do if you saw box B full (and never mind the case where box B is empty).
Predicting what you'd do if you saw box B empty (and never mind the case where box B is full).
Predicting what you'd do in both cases, and filling box B if and only if you'd one-box in both of them.
Or variations of those. There's no circularity when he only makes such "conditional" predictions.
He could use the same algorithms in the non-transparent case, and they would reduce to the normal newcomb's problem usually, but prevent you from doing any tricky business if you happen to bring an X-ray imager (or kitchen scales) and try to observe the state of box B.