The fact that the Schrödinger and the Dirac equations for the wave function of an electron are differential equations of the first order with respect to time, while in classical theories differential equations of the second order are common, has necessitated a slightly different setup of the canonical theory in both fields. Under influence of the classical methods, second-order equations are also often used in the quantum theory of particles of integer spin, thus causing a difference between the treatment of Fermi-Dirac and of Einstein-Bose particles.

It is shown here that conformity between classical and quantum-mechanical methods can be achieved easily by use of first-order equations throughout, thus avoiding a superfluous distinction between integral and half-odd-integral spin fields. The classical theory of a point charge in an electromagnetic field of force is set up here from this point of view.

• Received 21 April 1948

DOI:https://doi.org/10.1103/PhysRev.74.779