In the Bayesian approach to quantum mechanics, probabilities—and thus quantum states—
represent an agent's degrees of belief, rather than corresponding to objective properties of
physical systems. In this paper we investigate the concept of certainty in quantum
mechanics. Particularly, we show how the probability-1 predictions derived from pure
quantum states highlight a fundamental difference between our Bayesian approach, on the
one hand, and Copenhagen and similar interpretations on the other. We first review the …