D O C U M E N T 5 4 F E B R U A R Y 1 9 2 2 7 5 Because I have to write an annual progress report that contains as its essential core the fact of your (twice yearly) presence in Leyden.—Your presence here, even if you “do absolutely nothing,” means for all of us (especially also for Onnes and me and our older students) enormously much. More than you can fathom, because each time, your presence here triggers a long-lasting series of discussions [e.g., if you come in April, then I’ll tell you about what I learned at Bohr’s[3] —you’ll respond— this again provides a lot of inspiration to all of us—and this is just one of 5–6 topics that are waiting for you].[4] So if you do come (twice a year) I can bona fide and happily recount all that came out of it in my progress report.—And nobody then mindlessly asks, “What did he do?”—If you don’t come, however, then it would put me in a pickle. Quite apart from how sorry I would feel not to see you. Understand me correctly: Neither Lorentz nor Onnes nor anyone else here would permit themselves to say such a thing to you—indeed, I think they are so very Dutch that they even understand the art of not just not saying it, but even ban- ishing it from their thoughts.—And everyone here loves you so much and values the benefits of your presence here so highly (it really is very high!—Plain fact.) that one would rather have you here 1/2 the time than not at all. But I am allowed to tell you that it would put me in a pickle to have to explain an absence during spring 1922 in my next report, as I already had to do so for spring 1921 (with your Amer- ica trip). Oh, please, please, Einstein, come to Leyden, for ex. for the second half of April, let me tell you about what Bohr taught me, have a look at what Onnes is doing, lis- ten to what Lorentz is bringing home from America—[Onnes probably wrote you that indicates no resistance].[5] In truth, you will not regret it and no one wants more from you! Also, at the !end of April! you certainly won’t have the feeling that momentarily confuses you: “But I was in Leyden just recently!” Every time that you are here, you don’t find it that bad at all! Greetings to all of you, yours, P. Ehrenfest + - lead tin lead (helium temperature)