Yeah well it is certainly a good argument against that. The title of the thread is "is induction unformalizable" which point I'm unconvinced of.
If I were to formalize some kind of prior, I would probably use a lot of epsilons (since zero is not a probability); including an epsilon for "things I haven't thought up yet." On the other hand I'm not really an expert on any of these things so I imagine Wei Dai would be able to poke holes in anything I came up with anyway.
There's no general way to have a "none of the above" hypothesis as part of your prior, because it doesn't make any specific prediction and thus you can't update its likelihood as data comes in. See the discussion with Cyan and others about NOTA somewhere around here.
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