For example, if there were such a thing as a gene for optimism versus pessimism, you
might believe that you had an equal chance of inheriting your mother’s optimism gene or your
father’s pessimism gene. You might further believe that your sister had the same chances as
you, but via an independent draw, and following Mendel’s rules of inheritance. You might
even believe that humankind would have evolved to be more pessimistic, had they evolved
in harsher environments. Beliefs of this sort seem central to scientific discussions about the
origin of human beliefs, such as occur in evolutionary psychology. [...]
Consider, for example, two astronomers who disagree about whether the
universe is open (and infinite) or closed (and finite). Assume that they are both aware of
the same relevant cosmological data, and that they try to be Bayesians, and therefore want
to attribute their difference of opinion to differing priors about the size of the universe.
This paper shows that neither astronomer can believe that, regardless of the size of the
universe, nature was equally likely to have switched their priors. Each astronomer must
instead believe that his prior would only have favored a smaller universe in situations where
a smaller universe was actually more likely. Furthermore, he must believe that the other
astronomer’s prior would not track the actual size of the universe in this way; other priors
can only track universe size indirectly, by tracking his prior. Thus each person must believe
that prior origination processes make his prior more correlated with reality than others’
priors.
As a result, these astronomers cannot believe that their differing priors arose due to the
expression of differing genes inherited from their parents in the usual way. After all, the usual
rules of genetic inheritance treat the two astronomers symmetrically, and do not produce
individual genetic variations that are correlated with the size of the universe.
This paper thereby shows that agents who agree enough about the origins of their priors
must have the same prior.
Hanson saying the same: