Recently, some have challenged the idea that there are genuine norms of diachronic
rationality. Part of this challenge has involved offering replacements for diachronic principles.
Skeptics about diachronic rationality believe that we can provide an error theory for it by
appealing to synchronic updating rules that, over time, mimic the behavior of diachronic norms.
In this paper, I argue that the most promising attempts to develop this position within the
Bayesian framework are unsuccessful. I sketch a new synchronic surrogate that draws upon
some of the features of each of these earlier attempts. At the heart of this discussion is the
question of what exactly it means to say that one norm is a surrogate for another. I argue that
surrogacy, in the given context, can be taken as a proxy for the degree to which formal and
traditional epistemology can be made compatible.