The following appears on p. 10 of Jaynes' Probability Theory: The Logic of Science:
Evidently, then, it must be the most primitive axiom of plausible reasoning that two propositions with the same truth value are equally plausible. This might appear almost too trivial to mention, were it not for the fact that Boole himself (Boole, 1854, p. 286) fell into error on this point, by mistakenly identifying two propositions which were in fact different – and then failing to see any contradiction in their different plausibilities.
What is the particular example Jaynes is referring to?