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Full Idea
If the only objective constraints concern how we should change our credences, but what our initial credences should be is entirely subjective, then individuals with very bizarre opinions about the world will count as perfectly rational.
Gist of Idea
If the rules only concern changes of belief, and not the starting point, absurd views can look ratiional
Source
Samir Okasha (Philosophy of Science: Very Short Intro (2nd ed) [2016], 2)
Book Ref
Okasha,Samir: 'Philosophy of Science: very short intro (2nd ed)' [OUP 2016], p.35
A Reaction
The important rationality has to be the assessement of a diverse batch of evidence, for which there can never be any rules or mathematics.