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Liron 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.
Like PaulFChistiano said, the shortest accurate program isn't particularly useful, but its predictive model is more a priori probable according to the universal / Occamian prior.
It's really hard (and uncomputable) to discover, understand, and verify the shortest program that computes a certain input->prediction mapping. But we use the "shortest equivalent program" concept to judge which human-understandable program is more a priori probable.