4 4 0 D O C U M E N T 4 4 6 J A N U A R Y 1 9 2 7 to flow, simply because officially, that would be the most convenient and I am asked to make this clear to you, which I herewith wish to have the honor of being considered to have done. __________________________________ De Haas was operated on about one week ago after a very long, thorough pre- liminary examination.[3] As anticipated, only gall stones were found and they were cut out. The operation went well and, and as I hear from Mrs. De Haas,[4] De Haas is doing very satisfactorily. If you wrote them a couple of words, they would be enormously pleased! As Joffe, en route to California, was for the first time unable to visit me in Leyden,[5] I traveled to Paris to see him. My first visit to Paris! Immense impres- sion of this truly very special city. Despite not being able to speak French, I did have an excellent time with people there. I’m probably going to be traveling there often now. I’d give a lot to be able to be there with you once! Hadamard also asked me to visit him.[6] Wow, what a fine type of Jew he is!!!! I got a tremendously funny impression of a session of the Academy of Science. I’m terribly curious to see what[7] you’ve found out about the singular world- lines of the gravitational field.[8] I think it’s a completely wonderful, beautiful thing. As regards the purported condensation, […] firm in my opinion: I. That the (su- perproportional) occupation of the lowest quantum state with rising number of molecules is not allowed to be called “condensation,” because there is nothing that would IN ANY WAY be analogous to the transition from a purely one-phase sys- tem into a two-phase system upon exceeding a particular number of molecules.— II. That your approximation by integrals really and truly stops being permissible as soon as the increase in the number of molecules has come so far that the occupa- tions of the first, second, and third quantum states already differ strongly from one another, meaning, when that range has just been reached where you triumphantly (but incorrectly) proclaim: Do you see the condensation occurring at the lowest quantum state?! Point II is not as important as point I, because just this question of the NOT-two- PHASEDNESS is already clearly visible, without that integral approximation, which becomes invalid there, being employed. But we are the ones who are right, and not you! Period. And Pauli[9] also conceded that we’re right: when I wrote to him about it. In the coming days I shall take the liberty of sending you a short quantum matter that I just discovered in the past few days.[10] I still just have to check whether ev- erything is in good order. But I know that you’ll like it if it stays the way I now have