1945 WPAULI is not yet sufficiently clear. Already now we can say, however, that these in- teractions are fundamentally different from electromagnetic interactions The comparison between neutron-proton scattering and proton-proton scattering even showed that the forces between these particles are in good approximation the same, that means independent of their electric charge. If one had only to take into account the magnitude of the interaction energy, one should therefore expect a stable di-proton or 2He(M=2,Z= 2)with forbidden by the exclusion principle in accordance with experience, because his state would acquire a wave function symmetric with respect to the two protons. This is only the simplest example of the application of the exclusion principle to the structure of compound nuclei, for the understanding of which this principle is indispensable, because the constituents of these heavier nuclei, the protons and the neutrons, fullfil In order to prepare for the discussion of more fundamental questions, we want to stress here a law of Nature which is generally valid, namely, the onnection between spin and symmetry class. A half-integer value of the spin quantum number is always connected with antisymmetrical states(exclusion prin ciple), an integer spin with symmetrical states. This law holds not only for pro- tons and neutrons but also for protons and electrons. Moreover, it can easily be seen that it holds for compound systems, if it holds for all of its constit uents. If we search for a theoretical explanation of this law, we must pass to the discussion of relativistic wave mechanics, since we saw that it can cer tainly not be explained by non-relativistic wave mechanics We first consider classical fields, which, like scalars, vectors, and tensors transform with respect to rotations in the ordinary space according to a one- valued representation of the rotation group. We may, in the following, call such fields briefly u one-valued fields So long as interactions of different kinds of field are not taken into account, we can assume that all field com- ponents will satisfy a second-order wave equation, permitting a superpo tion of plane waves as a general solution. Frequency and wave number of these plane waves are connected by a law which, in accordance with De Broglie's fundamental assumption, can be obtained from the relation be tween energy and momentum of a particle claimed in relativistic mechanics by division with the constant factor equal to Planck's constant divided by 2T. Therefore, there will appear in the classical field equations, in general,a new constant H with the dimension of a reciprocal length, with which the36 1945 W.PAUL I is not yet sufficiently clear. Already now we can say, however, that these in￾teractions are fundamentally different from electromagnetic interactions. The comparison between neutron-proton scattering and proton-proton scattering even showed that the forces between these particles are in good approximation the same, that means independent of their electric charge. If one had only to take into account the magnitude of the interaction energy, one should therefore expect a stable di-proton or :He (M = 2, Z = 2) with nearly the same binding energy as the deuteron. Such a state is, however, forbidden by the exclusion principle in accordance with experience, because this state would acquire a wave function symmetric with respect to the two protons. This is only the simplest example of the application of the exclusion principle to the structure of compound nuclei, for the understanding of which this principle is indispensable, because the constituents of these heavier nuclei, the protons and the neutrons, fullfil it. In order to prepare for the discussion of more fundamental questions, we want to stress here a law of Nature which is generally valid, namely, the connection between spin and symmetry class. A half-integer value of the spin quantum number is always connected with antisymmetrical states (exclusion prin￾ciple), an integer spin with symmetrical states. This law holds not only for pro￾tons and neutrons but also for protons and electrons. Moreover, it can easily be seen that it holds for compound systems, if it holds for all of its constit￾uents. If we search for a theoretical explanation of this law, we must pass to the discussion of relativistic wave mechanics, since we saw that it can cer￾tainly not be explained by non-relativistic wave mechanics. We first consider classical fields20 , which, like scalars, vectors, and tensors transform with respect to rotations in the ordinary space according to a one￾valued representation of the rotation group. We may, in the following, call such fields briefly « one-valued » fields. So long as interactions of different kinds of field are not taken into account, we can assume that all field com￾ponents will satisfy a second-order wave equation, permitting a superposi￾tion of plane waves as a general solution. Frequency and wave number of these plane waves are connected by a law which, in accordance with De Broglie’s fundamental assumption, can be obtained from the relation be￾tween energy and momentum of a particle claimed in relativistic mechanics by division with the constant factor equal to Planck’s constant divided by 2p. Therefore, there will appear in the classical field equations, in general, a new constant m with the dimension of a reciprocal length, with which the