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paulfchristiano comments on A note on the description complexity of physical theories - Less Wrong
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When there is a testable physical difference between hypotheses, we want the one that makes the correct prediction.
When there is no testable physical difference between hypotheses, we want to use the one that makes it easiest to make the correct prediction. By definition, we can never get a prediction that wouldn't have happened were we using the other hypothesis, but we'll get that prediction quicker. Neither hypothesis can be said to be 'the way the world really is' because there's no way to distinguish between them, but the simpler hypothesis is more useful.
The length of the program description is not really the measure of how easy it is to make a correct prediction. In fact, the shortest program for predicting is almost never the one you should use to make predictions in practice, precisely because it is normally quite slow. It is also very rarely the program which is easiest to manipulate mentally, since short programs tend to be very hard for humans to reason about.