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, but the Bayesian apparatus doesn't really give us a means of determining the "best" among those. I wrote about these topics here before. If you want the one-word summary, the area is a mess.
Thanks for the links (and your post!), I now have a much clearer idea of the depths of my ignorance on this topic.
I want to believe that there is some optimal general prior, but it seems much more likely that we do not live in so convenient a world.
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