RichardKennaway comments on Open Thread: September 2009 - Less Wrong
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No, let me try nailing this jelly to the wall once again. The definition-only-up-to-a-constant is a weakness of MDL, but this weakness isn't relevant to my question at all! Even if we had some globally unique variant of MDL derived from some nice mathematical idea, learning theory still doesn't use description lengths, and would be perfectly happy with rules that have long descriptions as long as we delineate a small set of those rules. To my mind this casts doubt on the importance of MDL.
I think its uncomputability already does that. When you make a computable version by limiting attention to some framework of descriptive capabilities smaller than universal computation, different choices of that framework will give you different measures of simplicity. What is simple in one framework may seem elaborate and baroque in another. Or as some military strategist once put it:
"To the foot-soldier, the strategy of a general may seem obscure, shrouded in shadows and fog, but to the general himself, his way is as plain as if he were marching his army down a broad, straight highway."