Let's consider the hydrogen atom with one electron. If we observe the energy the result is always one of the eigenvalues even if the statefunction is arbitrary. I accept that. But why is the atom always treated as if the electron is in one of the eigenstates? Why is the statefunction always supposed to be an eigenstate? Is that a postulate? Or is the electron not always supposed to be in an eigenstate? Does an arbitrary statefunction evolve into an eigenstate?