"What is the empirical content of quantum mechanics? Or how does the wave function assigned to a relate to the results of measurements on the system? The well-known connection rule is the , which has been precisely tested by experiments. It says that a (projective) measurement of an A on a system with the wave function |ψ⟩ will randomly obtain one of the of A, and the probability of obtaining an eigenvalue ai is given by |⟨ai|ψ⟩|2, where |ai⟩ is the corresponding to the eigenvalue ai."