 There are a number of theorems that circumscribe the scope of possible hidden-variables theories. The most natural thought would be to seek a theory that assigns to all quantum observables definite values that are merely revealed upon measurement, in such a way that any experimental procedure that, in conventional quantum mechanics, would count as a “measurement” of an observable yields the definite value assigned to the observable. Theories of this sort are called noncontextual hidden-variables theory. It was shown by Bell (1966) and Kochen and Specker (1967) that there are no such theories for any system whose Hilbert space dimension is greater than three (see the entry on  the Kochen-Specker theorem).