Quantum mechanics was initiated with the object of allowing only observable concepts to enter into the theory. The new mechanics has, however, inherited the old difficulty with the conservation laws involved in beta decay, and this led Pauli, about 1931, to introduce the idea of the neutrino, with the object of reconciling the facts of beta decay with the conservation laws. The neutrino, as it was proposed by Pauli and as accepted today, is a particle devoid both of mass and charge, but having a spin 1/2 and obeying the Fermi statistics. In 1934 Fermi founded his theory of beta disintegration based on the neutrino. The Fermi theory assumes that this hypothetical particle is emitted simultaneously with the disintegrating electron and carries off the excess energy, thereby reconciling the conservation laws with beta disintegration. It will be remembered that the continuous distribution in energy of the beta particles is such that, except at the maximum energy end point of the beta spectra, there is more energy available from the disintegration than is emitted in the form of beta particles.