Are the priors of subjective Bayesianism subjective?

Visual and auditory perception are not subject to rational evaluation. Nobody perceives visually or auditorily in a rational or irrational way. Nonetheless, perception is subject to evaluation. One’s perceptual faculties could function superlatively, adequately or inadequately. The mere fact that they are not subject to rational evaluation does not imply subjectivism about their functioning.

But now consider a Bayesian view on which:

1. Our priors are not subject to rational evaluation.

That view is known in the literature as “subjective Bayesianism”. But if we take seriously the lesson from perception, we should be sceptical of the inference from (1) to:

2. Our priors are merely subjective.

I have to confess to not taking this point seriously in the past, having been misled by the phrase “subjective Bayesianism” and by things I heard from subjective Bayesians.

What might a theory look like on which our priors are subject to evaluation but not rational evaluation? We could take our priors to be a kind of “probabilistic perception” of patterns in the world, a perception that is genetically and/or socially mediated. Such perceptions can be better or worse, just as the person who is looking at a horse and their visual system classifies it as 95% likely to be cat and 5% likely to be a horse is doing less well perceptually than one whose system makes the opposite classification. For instance, someone who has a prior close to 1 for the law of gravitation being an inverse 3.00001th power law is doing less well than someone who has a moderately high prior for it it being an inverse cube law and a moderately high prior for it being an inverse square law.

But if we take the perception analogy seriously, we get this question: What are we “perceiving” with our priors? Maybe something like facts about the sorts of laws worlds like ours have?