QBians Do Not Exist

TL;DR

This paper rebuts John Earman's portrayal of QBism by arguing that his 'QBians' caricature misrepresents the actual program. It clarifies QBism's core ideas: probabilities as personal credences, measurement outcomes as agent experiences, and a POVM-based, normatively guided formalism that replaces a fixed lattice of quantum propositions. The authors demonstrate that Earman's lattice-centric criticisms and misreadings of updates (Lüders, Kraus maps) miss the QBist view, which treats time evolution and measurement as coherent changes in an agent's beliefs rather than ontic state changes. By tracing QBism's historical development and emphasizing its two levels of personalism, the paper highlights QBism's internal consistency and its distinctive stance on measurement, probability, and the Born rule, and contends that the label 'QBians' is a misnomer with little bearing on the actual theory.

Abstract

We remark on John Earman's paper ``Quantum Bayesianism Assessed'' [The Monist 102 (2019), 403--423], illustrating with a number of examples that the quantum ``interpretation'' Earman critiques and the interpretation known as QBism have almost nothing to do with each other.