 More specifically, Gleason's theorem rules out hidden-variable models that are "noncontextual".Any hidden-variable model for quantum mechanics must, in order to avoid the implications of Gleason's theorem, involve hidden variables that are not properties belonging to the measured system alone but also dependent upon the external context in which the measurement is made.This type of dependence is often seen as contrived or undesirable; in some settings, it is inconsistent with special relativity.