  Given an ontological model of a quantum system, a "genuine measurement," as
opposed to a quantum measurement, means an experiment that determines the value
of a beable, i.e., of a variable that, according to the model, has an actual
value in nature before the experiment. We prove a theorem showing that in every
ontological model, it is impossible to measure all beables. Put differently,
there is no experiment that would reliably determine the ontic state. This
result shows that the positivistic idea that a physical theory should only
involve observable quantities is too optimistic.
