 Observable quantities are represented by self-adjoint operators \(B\) on the Hilbert space containing the possible states of the system. The associated eigenvalue equations \(B\ket{b_k} = b_k \ket{b_k}\) and the corresponding eigenmanifolds (the linear manifolds spanned by the eigenvectors associated to a given eigenvalue, also called eigenspaces) play a basic role for the predictive content of the theory. In fact: