Apologies if this has already been covered elsewhere, but isn't a prior just a belief? The prior is by definition whatever it was rational to believe before the acquisition of new evidence (assuming a perfect Bayesian, anyway). I'm not quite sure what you mean when you propose that a prior could be wrong; either all priors are statements of belief and therefore true, or all priors are statements of probability that must be less accurate than a posterior that incorporates more evidence.
I suspect that there are additional steps I'm not considering.
The prior is by definition whatever it was rational to believe before the acquisition of new evidence (assuming a perfect Bayesian, anyway).
Nope, this isn't part of the definition of the prior, and I don't see how it could be. The prior is whatever you actually believe before any evidence comes in.
If you have a procedure to determine which priors are "rational" before looking at the evidence, please share it with us. Some people here believe religiously in maxent, others swear by the universal prior, I personally rather like reference priors,...
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