probabilities on which it is based.

One of the greatest achievements of quantum Bayesianism is its response to the challenge of not being able to make sense of talk about “unknown quantum states”, even though such talk has a prominent role, for instance in quantum state tomography. From the quantum Bayesian point of view, the notion of an “unknown quantum state” makes no sense: any quantum state is always *someone’s* quantum state, assigned to a physical system by an agent and representing her subjective degrees of belief. The result on which quantum Bayesians ground their response is the so-called *quantum de Finetti theorem*, which makes it understandable why different agents who register the same measured data generally come to agreement in their assignment of quantum states in the long run, even if the states they start out with are very different. The main presupposition used is that the agents (subjectively) judge the states of the sequence of measured systems to be exchangeable, roughly meaning that for all agents both the order of measured events and the number of measurements witnessed are irrelevant. As demonstrated by Caves, Fuchs, and Schack, this suffices to guarantee that “the updated probability $P(\rho|D_K)$ becomes highly 