 Following a largely sympathetic sketch of QBism, Zwirn (forthcoming, §10) highlights ways in which some of its key notions remain unclear. Regarding quantum mechanics as an extension of subjective probability theory, QBists (DeBrota and Stacey (2019), see  Other Internet Resources)  reject the demand to provide a reductive definition of the notion of an agent. Zwirn presses this demand because in this context the agent is not merely a passive witness: “It is the interaction between an agent and the external world that creates a result. Without agent, there is no result.”