 However, this probability assignment does not correspond to any valid density operator.By introducing a probability distribution over the possible values of formula_23, a hidden-variable model for a qubit that reproduces the predictions of quantum theory can be constructed.Gleason's theorem motivated later work by John Bell, Ernst Specker and Simon Kochen that led to the result often called the Kochen–Specker theorem, which likewise shows that noncontextual hidden-variable models are incompatible with quantum mechanics.