You're right that prior probability gets very, very messy. It's a bit too abstract to actually be helpful to us.

You cannot escape the necessity of dealing with priors, however messy they are.

So, then, all we can do is look at the evidence we do have.

The available evidence supports an infinite number of hypotheses. How do you decide which ones to consider? That is your prior, and however messy it may be, you have to live with it.