7 6 D O C U M E N T 6 8 S E P T E M B E R 1 9 2 5 68. From Paul Ehrenfest Leyden, 16 September 1925 Dear Einstein, To my happy surprise Bohr came to see us completely unexpectedly the day be- fore yesterday with his especially lovely wife and stayed with us for two days.[1] He is on a short trip to recuperate, which he as well as his wife very badly needed. You can imagine how very delighted I was, and Lorentz also.[2] They are gone again now, unfortunately. To Belgium. He is now wrestling enormously with the quantum problems and wants to speak with you in particular, more than with any- one else. It is so important to him to know how much you have encountered the same profound problems as he has. I know that no living human has peered so deeply into the veritable abysses of quantum theory as the two of you and that no- body apart from you two really sees how necessary completely radical new con- cepts are. When I told Bohr that you are going to be here for the Lorentz jubilee,[3] he was immediately resolved on coming here as well—on the one hand, in order to honor Lorentz on the other hand, to be able to spend at least one week together with you. I hope you also approve of this. I just have to ask you please to ¢secure² grant me the same unconditional authority for your isolation from visitors[4] as Bohr requested of me, because Bohr is very, very easily worn out. But he has a quite extraordinary desire to see you. You mustn’t think that I thought this up but he really is so modest and has such an enormous respect for you and Lorentz that he immediately and gladly grasped the opportunity to meet you here, though he would hardly have gathered the resolve to invite you to see him!!! Incidentally, I believe that the two of you could not be as isolated either in Berlin or in Copenha- gen as in my home, provided I receive the necessary isolation authorization from both of you. You would then be staying with us, next to each other alone upstairs (notabene: with a new, good bed!!! I was horrified to learn only now how bad the bed there was, which you slept on last time!) and taking peaceful walks in fine weather.— Now, please let me know, if possible right away, what day you intend to come and how long, at most, you can stay with us.[5] Please, please, for as long as possible!!! The Lorentz event is the 11th of December—a Friday.— You under- stand what a great experience it would be for me to hear the two of you discussing the puzzles of quantum theory together—but you can rely on my almost always leaving the two of you alone. It is of particular interest to me what the discussion between you and Bohr will yield regarding the experiments you are always plan- ning at the boundary between “waves and corpuscles.”[6] Whereas I expect that you will share very similar feelings as regards the general puzzles, fruitful conflicts be- tween the two of you are likely to come to light in this specific area.— So, you will