1945 W. PAULI the electron. The gap was filled by Uhlenbeck and Goudsmit's idea of elec tron spin, which made it possible to understand the anomalous Zeeman effect simply by assuming that the spin quantum number of one electron is ual to y2 and that the quotient of the magnetic moment to the mechanical angular moment has for the spin a value twice as large as for the ordinary orbit of the electron. Since that time, the exclusion principle has been closely con- nected with the idea of spin. Although at first I strongly doubted the correct- ness of this idea because of its classical-mechanical character, I was finally converted to it by Thomas calculations on the magnitude of doublet slitting. On the other hand, my earlier doubts as well as the cautious ex pression u classically non-describable two-valuedness m experienced a certain verification during later developments, since Bohr was able to show on the basis of wave mechanics that the electron spin cannot be measured by clas- sically describable experiments(as, for instance, deflection of molecular beams in external electromagnetic fields)and must therefore be considered as an essentially quantum-mechanical property of the electron The subsequent developments were determined by the occurrence of the new quantum mechanics. In 1925, the same year in which I published my paper on the exclusion principle, De Broglie formulated his idea of matter waves and Heisenberg the new matrix-mechanics, after which in the next year Schrodingers wave mechanics quickly followed. It is at present un- necessary to stress the importance and the fundamental character of these discoveries, all the more as these physicists have themselves explained, here in Stockholm, the meaning of their leading ideas. Nor does time permit me to illustrate in detail the general epistemological significance of the new discipline of quantum mechanics, which has been done, among others, in a number of articles by Bohr, using hereby the idea of complementarity as a new central concept. I shall only recall that the statements of quantum me- chanics are dealing only with possibilities, not with actualities. They have the form This is not possible p or< Either this or that is possible m, but they can never say That will actually happen then and there p. The actual observa- tion appears as an event outside the range of a description by physical laws and brings forth in general a discontinuous selection out of the several pos- sibilities foreseen by the statistical laws of the new theory. Only this renounce- ment concerning the old claims for an objective description of the physical phenomena, independent of the way in which they are observed, made it possible to reach again the self-consistency of quantum theory, which ac30 1945 W.PAUL I the electron. The gap was filled by Uhlenbeck and Goudsmit’s idea of elec￾tron spin6 , which made it possible to understand the anomalous Zeeman effect simply by assuming that the spin quantum number of one electron is equal to ½ and that the quotient of the magnetic moment to the mechanical angular moment has for the spin a value twice as large as for the ordinary orbit of the electron. Since that time, the exclusion principle has been closely con￾nected with the idea of spin. Although at first I strongly doubted the correct￾ness of this idea because of its classical-mechanical character, I was finally converted to it by Thomas’ calculations7 on the magnitude of doublet splitting. On the other hand, my earlier doubts as well as the cautious ex￾pression « classically non-describable two-valuedness » experienced a certain verification during later developments, since Bohr was able to show on the basis of wave mechanics that the electron spin cannot be measured by clas￾sically describable experiments (as, for instance, deflection of molecular beams in external electromagnetic fields) and must therefore be considered as an essentially quantum-mechanical property of the electron8,9 . The subsequent developments were determined by the occurrence of the new quantum mechanics. In 1925, the same year in which I published my paper on the exclusion principle, De Broglie formulated his idea of matter waves and Heisenberg the new matrix-mechanics, after which in the next year Schrödinger’s wave mechanics quickly followed. It is at present un￾necessary to stress the importance and the fundamental character of these discoveries, all the more as these physicists have themselves explained, here in Stockholm, the meaning of their leading ideas10. Nor does time permit me to illustrate in detail the general epistemological significance of the new discipline of quantum mechanics, which has been done, among others, in a number of articles by Bohr, using hereby the idea of « complementarity » as a new central concept II. I shall only recall that the statements of quantum me￾chanics are dealing only with possibilities, not with actualities. They have the form « This is not possible » or « Either this or that is possible », but they can never say « That will actually happen then and there ». The actual observa￾tion appears as an event outside the range of a description by physical laws and brings forth in general a discontinuous selection out of the several pos￾sibilities foreseen by the statistical laws of the new theory. Only this renounce￾ment concerning the old claims for an objective description of the physical phenomena, independent of the way in which they are observed, made it possible to reach again the self-consistency of quantum theory, which ac-