Lev Landau

“ We see that no special properties of the electron have been taken into account, so that our reasoning applies to all analogous problems where the external forces have a potential. To speak of a rotation here is completely meaningless, since the equations are strictly valid for a single point. The notion of a rotating point would be strange!
We can generalize the problem with the help of the usual four-dimensional space. As in classical wave mechanics, the idea of a coordinate space with any number of dimensions can be introduced. ”

“ In order to set up the desired equations, we resort to the methods of ordinary wave mechanics. From these we choose the method of the Lagrangian function, because this is the easiest way to calculate the basic quantities of the theory, such as the "current vector", "energy tensor" and others. The fundamental role is played by the velocity vector corresponding to the operator [6] ( is the four potential of the external electromagnetic field) . As is known, this suffices to set up the equations without using other operators. ”

“ Although Darwin [3] succeeded in setting up the correct equations by introducing not only a scalar but also a -vector although the coefficients remained unfounded for him and therefore relativistic invariance was not achieved. The same applies, of course, to Jordan's quaternion method. [4]
It seems, therefore, that the magnetic form of the electron has nothing to do with rotation, but represents a phenomenon which has it origin much more deeply in the nature of things. ”