I provide operational postulates for quantum theory. These involve certain operational notions. Systems come in different types, a, b, c, . . .. A maximal set of distinguishable states is any set containing the maximum number, Na, of states for which there exists some measurement, called a maximal measurement, which can identify which state from the set we have in a single shot. A maximal effect is associated with each result of a maximal measurement. An informational face is the full set of states that give rise only to some subset of outcomes of some maximal measurements (it corresponds to constraining the system to have some reduced information carrying capacity). States are represented by vectors whose Ka entries are probabilities. A set of states is said to be non-flat if it is a spanning subset of some informational face. A filter is a transformation that passes unchanged those states in a given informational face while blocking those states in the complement informational face (that would give rise only to outcomes in the complement outcome set of the maximal measurement). Classical probability theory and quantum theory are the only two theories consistent with the following set of postulates.P1 Logical sharpness. There is a one-to-one map between pure states and maximal effects such that we get unit probability.P2 Information locality. A maximal measurement is effected on a composite system if we perform maximal measurements on each of the components (or, equivalently, N ab = NaN b ).P3 Tomographic locality. The state of a composite system can be determined from the statistics collected by making measurements on the components (or, equivalently, K ab = KaK b ).
P4′ Permutability. There exists a reversible transformation on any system effecting any given permutation of any given maximal set of distinguishable states for that system. P5 Sturdiness. Filters are non-flattening.To single out quantum theory we need only add any requirement that is inconsistent with classical probability theory and consistent with quantum theory.