D O C U M E N T 2 5 8 A P R I L 1 9 2 6 2 7 3 258. To George Y. Rainich [Berlin,] 18 April 1926 Dear Mr. Rainich, I hurry to answer your letter,[1] happy that you struggle with the same questions as I myself have for such a long time, to no avail. The cardinal question is of course whether one should think of electricity as continuous or made up of singularities. The latter option seems easier at first sight, since one could then just stick with the Maxwell equations without adding anything to them. But it seems certain to me that one would then have to dispense with not just an explanation for the equality of the numerical values for electric charges and masses of the same sign. Further- more, it won’t be possible to arrive at a law of motion for electric charges, except by arbitrary assumptions with which one would not come any closer to the truth.[2] I am convinced that one could find a strict solution on the basis of the gravitational equations + Maxwell equations, which would represent the case of two electrons at rest (as singularities).[3] For the case in which the particles in question have no elec- tric charge this has already been shown by Weyl and Levi-Civita (special case of axial symmetry).[4] This would prove that your plan cannot be carried out. However, it must also be conceded that all attempts to treat electricity as a con- tinuum have failed up to now. Nevertheless, I am convinced that this is the path of the future, that the law of motion of electric particles can be obtained in this way only the same is true regarding the aim of understanding the constitution of these particles. Regarding your expression for the singularity,[5] I would like to comment that corresponds to a magnetic point mass, whose existence you surely wish to exclude. Therefore, you want to set You thus see that in such a theory the nonexistence of magnetic masses must be specifically presumed, which is also unsatisfactory. Respectfully yours, A. Einstein xμdyν ³fμνd 0 ³fμνdxμdyν = ρ2 A xμdyν]2. ³rμνd [ = =