of [@lucien] and [@barrett]. However, we should note that there is nothing very novel in the framework itself, and that numerous formalisms have been developed over the years, intended to be operational generalizations of classical and quantum theory (see e.g., [@lucien; @dariano; @convex1; @convex2]).

Single systems
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One way of specifying the state of a system would be to give an exhaustive list of the outcome probabilities for every possible measurement. However, as Hardy points out [@lucien], most physical theories have enough structure that it is not necessary to give such an exhaustive list in order to specify the state fully. Instead, for each type of system, assume that its state can be completely characterised by the outcome probabilities for some finite subset of all possible measurements. Following Hardy, we refer to the subset chosen to represent the state as *fiducial* measurements. If the outcome probabilities for the fiducial measurements are known, then the state is known, and the outcome probabilities for any other measurement can be inferred. For example, the state of a single qubit in quantum theory corresponds to a density operator on a 2-dimensional Hilbert space. But equally, it can be completely characterised by the outcome probabilities 