D O C U M E N T 2 2 0 R E S P O N S E T O B O V E T 1 8 1 Eddington’s arguments strike me as Mie’s theory does: It is a pretty frame but one absolutely cannot see how it has to be filled in.[4] Have you thought Kaluza’s approach through? To me it feels closest to reality but it does not yield a singular- ity-free electron either.[5] Permitting singularities does not seem the correct approach. I think—to really make some headway—one would have to find another general principle eavesdropped from nature. Cordial regards to you and your wife from your A. Einstein. 220. Response to Ernest Bovet’s Question to Paul Langevin [Einstein 1922j] Dated 7 June 1922 Published 1 September 1922 In: Wissen und Leben 15 (1 September 1922): 902. Dear Sir,[1] Your “Question to Mr. Langevin”[2] stimulates me to a reply. Regarding the gen- eral questions of interest to you, relativity theory does not change the situation at all. This is because it is no more than an improvement and modification of the basis of the worldview of physical causality, without any alteration to the fundamental aspects. It is a kind of logical system for describing spatio-temporal events in which the mental noumena (the will, emotion, etc.) have no immediate place. To avoid a collision between the different sorts of “realities” that physics and psychology deal with, Spinoza or, resp., Fechner invented the doctrine of psychophysical parallel- ism which, quite frankly, fully satisfies me.[3] Physics is one possible way among many equally legitimate ones to arrange experiences into a certain order. The foun- dations of this system are freely chosen by us, and in particular, according to the aspect that best accommodates the given known facts with a minimum of hypoth- eses. Thus “belief” is not what is involved but free choice according to the view- point of logical completeness and adaptability to experience, which is expressed so well in the quoted statements by Henri Poincaré.