  We consider an arbitrary operator Q on H3 with three distinct eigenvalues q1, q2, q3, its eigenvectors |q1&gt;, |q2&gt;, |q3&gt;, and projection operators P1, P2, P3 projecting on the rays spanned by these vectors. Now, P1, P2, P3 are themselves observables (namely, Pi is a ‘yes-no observable’ corresponding to the question ‘Does the system have value qi for Q?’). Moreover, P1, P2, P3 are mutually compatible, so we can apply the Sum Rule and Product Rule, and thereby derive a constraint on the assignment of values  (Proof):