Abstract: QBism (pronounced cubism) is a foundational program for quantum mechanics premised on the idea that quantum probabilities should be understood as Bayesian probabilities—i.e., quantified degrees of belief or gambling attitudes. Philosophers hate it. “Wah, wah, wah, your quantum states aren’t real; they have to be because my philosophy says so!” What the philosophers have never appreciated (or perhaps cared about) is that this turn in thinking has motivated a significant number of theorems and constructions in quantum information science that might not have been discovered otherwise. In this talk, I will sketch QBism’s most ambitious project yet: Rewriting the quantum formalism so that it wears its Bayesian character on its sleeve. In the process, we will see that it leads to a very deep mathematical question related to Hilbert’s still-unsolved 12th problem and suggests a novel quantum measurement that could have a number of uses in quantum information science (and maybe metrology).