D O C U M E N T 2 5 6 A P R I L 1 9 2 6 2 7 1 would like to see Painlevé relinquish the choice of the negotiation venue to the envoy.[4] As soon as I hear more, I shall write you. In the meantime, kind regards to you, sincerely yours, A. Einstein 256. To Erwin Schrödinger [Berlin,] 16 April 1926 Dear Colleague, Mr. Planck showed me your theory[1] with justifiable enthusiasm, which I then also studied with much interest. An objection then occurred to me that I hope you can dismiss. If I have two systems that are not at all coupled ¢energetically², and is an energy value possible for quanta of the first system, is such a value for the second system, then must be that of a total system comprising both. But I do not see why your equation should express this property. So that you see what I mean, I shall set up another equation that would satisfy this condition: Then the two equations (valid for the phase space of the 1st system) ( " " " " " " " 2nd " ) have as a consequence valid for the combined q-space) For the proof, one only needs to multiply the equations by or , respectively, and add. would hence be a solution to the equation for the combination sys- tem, which belongs to . I unsuccessfully attempted to set up such a relation for your equation. E1 E2 E1 E2 + E = div gradϕ E2 h2( E Φ) – ------------------------ϕ + 0 = div gradϕ E Φ – h2 -------------ϕ - + 0 = div gradϕ1 E1 Φ1 – h2 ------------------ϕ1 - + 0 = div gradϕ2 E2 Φ2 – h2 ------------------ϕ2 - + 0 = div grad(ϕ1ϕ2) E1 E2)– + ( Φ1 Φ2) + ( h2 ------------------------------------------------------(ϕ1ϕ2) + 0, = ϕ2 ϕ1 ϕ1ϕ2 E1 E2 +