  This work stands at the beginning of attempts to construct a  “unified field theory” which subsequently were continued  by many, it seems to me, without decisive results. As is known, the  problem relentlessly occupied Einstein in particular, until his end.  … The strongest argument for my theory appeared to be that  gauge invariance corresponds to the principle of the conservation of  electric charge just as coordinate invariance corresponds to the  conservation theorem of energy-impulse. Later, quantum theory  introduced the Schrödinger-Dirac potential \(\psi\) of the  electron-positron field; the latter revealed an experimentally based  principle of gauge invariance which guaranteed the conservation of  charge and which connected the \(\psi\) with the electromagnetic  potentials \(\varphi_{i}\) in the same way that my  speculative theory had connected the gravitational potentials  \(g_{ik}\) with \(\varphi_{i}\), where, in addition, the \(\varphi_{i}\) are measured in known  atomic rather than unknown cosmological units. I have no doubts that  the principle of gauge invariance finds its correct place here and  not, as I believed in 1918, in the interaction of electromagnetism  and gravity.