xxxviii INTRODUCTION TO VOLUME 5 Einstein came to view the prewar years as his happiest, and that he was nos- talgic about the freedom of choice he had enjoyed in Switzerland and the playfulness and creativity which had inspired him during his Swiss Years. [1]"In deinem kurzen aktiven Dasein hast Du in kindlicher Freude dich ergötzt an allem was klar und gescheit war." (see Einstein to The Immortal Olympia Academy, 3 April 1953). [2]That is not to say, of course, that some of the letters presented in this volume have not been pub- lished elsewhere. In such cases, we have taken pains to cite the bibliographic source. We apologize if we have inadvertently overlooked some. [3]In one case, a postscript by Mileva is included because Einstein directly comments on it (Doc. 267). [4]See, e.g., Einstein's unsuccessful attempts to draw Wilhelm Ostwald's attention to his first pub- lished paper (Vol. 1, Docs. 92, 95, and 99). [5]See Doc. 34. [6]See the editorial note, "Einstein's 'Maschinchen' for the Measurement of Small Quantities of Electricity," pp. 51-55. [7]"Unverfrorenheit." See Einstein to Mileva Marie, 27 December 1901, Vol. 1, Doc. 127. [8]See Vol. 3, the editorial note, "Einstein's Lecture Notes," for more on his teaching activities in Zurich. A full overview of the courses Einstein taught in Bern, Zurich, and Prague can be found in the Calendar/Chronology in this volume. [9]Einstein nevertheless taught a fair number of courses at the University of Berlin (see Vol. 3, Appendix B, "Einstein's Academic Courses," for an overview). [10]See Doc. 267. [11]See Doc. 313, note 7, for more details. [12]See Doc. 166. [13]See Doc. 399. [14]See Doc. 247. [15]See Doc. 313. [16]"Mit der durch dies Prinzip im Bereich der physikalischen Weltanschauung hervorgerufenen Umwälzung ist an Ausdehnung und Tiefe wohl nur noch die durch Einführung des Copernikanischen Weltsystems bedingte zu vergleichen." (see An das Professorenkollegium der philosophischen Fakultät der k.k. deutschen Universität in Prag, before 21 April 1910, Cz-Ar, MKV/R, No. 101, Ein- stein Dossier). [17]See Doc. 199, note 9. [18]See Einstein to Heinrich Zangger, 7 July 1915. [19]See Doc. 224, note 3. The declaration was a condition for Einstein's appointment at the German University of Prague.