3 5 2 D O C U M E N T 3 5 6 A U G U S T 1 9 2 6 If one observes a resonator of energy E during such a short time τ that during this time the percentage change of E is infinitesimally small, then the energy changes 1) by spontaneous emission 2) by irradiation (work of the external radiation field exerted on the oscillator). The sign of the irradiation depends on the oscillator’s phase and for that reason is just as likely to be positive as negative. The theory is nothing other than a suitable translation of this train of thought into quantum theory. Accordingly, it is simply impossible to omit the negative irradia- tion and this is by no means an ad hoc postulate. Otherwise, the theory would not have satisfied me. I hope this reflection is convincing to you. Please let me know what you think of it. Until then, kind regards, your A. Einstein 356. To Paul Ehrenfest [Berlin,] 23 August 1926 Dear Ehrenfest, Mr. Grommer drew my attention to a paper by Klein.[1] It is probably the topic that what was supposed to have been discussed in Leyden.[2] Characteristics: Kaluza, Schrödinger, gen. theory of rel.— Please procure me an offprint, at least temporarily. I am toiling over Dirac.[3] This balancing on a dizzying path between genius and insanity is terrible. Nothing that one can grasp firmly in hand! Best wishes for a happy vacation to you and yours from your Einstein