A better prior is a worse (but not useless) prior plus some evidence.
You construct a usable prior by making damn sure that the truth has non-exponentially-tiny probability, such that with enough evidence, you will eventually arrive at the truth.
From the inside, the best prior you could construct is your current belief dynamic (ie. including how you learn).
From the outside, the best prior is the one that puts 100% probability on the truth.
A better prior is a worse (but not useless) prior plus some evidence.
You construct a usable prior by making damn sure that the truth has non-exponentially-tiny probability, such that with enough evidence, you will eventually arrive at the truth.
From the inside, the best prior you could construct is your current belief dynamic (ie. including how you learn).
From the outside, the best prior is the one that puts 100% probability on the truth.