D O C U M E N T 2 4 5 A P R I L 1 9 2 6 2 5 5 ity actually means. On its own, it means nothing more than a graphic representation and for that reason states nothing about the physics. I wanted to illustrate this thought most clearly by showing that the old th. of rel. can be represented in the Weyl space without any alteration to its physical content. If it would work to posit unified field equations that allow the electron to be their solution, that would be something new in physics. To achieve this, something other than a merely formal summary of Maxwell’s eqs. and the gravitational equations would have to be provided, though the content of those eqs. would have to be al- tered. That is also the problem you are working on and the one Weyl and Eddington naturally also mean. But a geometric description of electricity, on its own, does not lead directly to this goal yet. At most, it can be a tool in guessing the right equa- tions perhaps what seems to be the simplest from the point of view of Weyl’s ge- ometry is by chance also physically right. But that would be mere chance. The intention of my exposition was to counter the view that something would already be gained by a geometrical description of electricity. As long as the existing theories do not yield the electron as a solution, however, they are of no more use than my simple reformulation of the old th. of rel. Compared to other geometrical descriptions this reformulation does have the advantage that its displacement oper- ation can be physically realized.[8] If you permit me, I will soon send you the pertinent §§ of my exposition but I am not quite finished with it yet.[9] I am, with cordial thanks and regards, your Hans Reichenbach 245. From George Y. Rainich Johns Hopkins University, Baltimore, Md., U.S.A., 5 April 1926 Dear Mr. Einstein, I have received your kind letter of 8 March 1926[1] and gather from it that I did not express myself clearly enough in the note written for Physica.[2] Thank you for addressing this issue in your letter I believe that I have now completely understood your view, and now take the liberty to try to make my own clear to you. 1. It seems to me that the problem you indicate can be traced back to your goal of characterizing the properties of particles by the “field inside matter.” Obviously, that way you rob yourself of the possibility of studying the relations between two particles directly and are obliged to try to find these relations by comparison of the findings stemming from studying the respective particles independently. In my