  Let us consider first an episode in which the relation between theory and experiment was clear and straightforward. This was a “crucial” experiment, one that decided unequivocally between two competing theories, or classes of theory. The episode was that of the discovery that parity, mirror-reflection symmetry or left-right symmetry, is not conserved in the weak interactions. (For details of this episode see Franklin (1986, Ch. 1) and  Appendix 1).  Experiments showed that in the beta decay of nuclei the number of electrons emitted in the same direction as the nuclear spin was different from the number emitted opposite to the spin direction. This was a clear demonstration of parity violation in the weak interactions. 