A convenient configuration space X is such that each point x produces some unique value of the observable Q. For discrete X it means that all elements of the standard basis are eigenvectors of Q. Then ψ ψ ( x ) is the probability amplitude for the eigenstate | x ⟩. If it corresponds to a non- degenerate eigenvalue of Q, then | ψ ψ ( x ) | 2 gives the probability of the corresponding value of Q for the initial state |Ψ⟩.