I N T R O D U C T I O N T O V O L U M E 1 5 l i III. Geometrization and Kaluza-Klein Theory We learn from the correspondence with Rainich that Einstein would consider a unification of gravity and electromagnetism satisfactory only if it could account for the quantum properties of matter. Likewise, we learn from Einstein’s correspon- dence with Hans Reichenbach (Docs. 224, 229, 234, 239, 244, 249) that another aim often associated with Einstein’s project of finding a unified field theory— namely, that of “geometrization”—was in fact not one of his desiderata. This particular part of Reichenbach’s correspondence with Einstein was trig- gered by Einstein’s latest attempt at a unified field theory (Einstein 1925t [Doc. 17]). In March 1926, Reichenbach (see Illustration 27) wrote that all the re- cent attempts at a unified field theory felt somewhat “artificial” to him (Doc. 224) possibly to his surprise, Einstein swiftly agreed (Doc. 230). Emboldened, Reichen- bach sent a seven-page manuscript (enclosed with Doc. 235) in which he argued that the geometric interpretation of electricity in previously suggested unified field theories was only a “visualization” of the physics, and not itself something physi- cal. He aimed to show this explicitly by providing a reformulation of the coupled Einstein and Maxwell equations, and especially of the Lorentz force law, in which both the electromagnetic and the gravitational field appear equally “geometrical” by being absorbed into a generalized affine connection whose geodesics are to be traced by charged particles. Einstein’s comments on the manuscript engaged with the details, rather than with the message. He had found more than one “fly in the soup.” His most import- ant objection was that one affine connection allows only for one type of particle, that is, one ratio of electric charge to mass, to move on its geodesics (Doc. 239). Reichenbach humbly replied that Einstein had misunderstood him as trying to set up a new unified field theory, whereas in fact he had only intended to give a repre- sentation of existing physics, one that showed that geometricity (in the sense de- veloped by Reichenbach) is a matter of mathematical representation rather than of physical content (Doc. 244). Einstein could identify with this latter message. He wrote that it “is wrong to think that ‘geometrization’ is something essential. It is only a kind of crutch for discovering numerical laws. Whether one links ‘geomet- rical’ intuitions with a theory is an inessential private matter” (Doc. 249). Thus, Einstein and Reichenbach agreed to agree. But their reasons for the rejection of ge- ometrization as a signal aim of general relativity and unified field theories were rather different.[16]