I profited from earlier reviews of the field, or of parts of it, by Pauli (, Section V); Ludwig ; Whittaker (, pp. 188–196); Lichnerowicz ; Tonnelat (, pp. 1–14); Jordan (, Section III); Schmutzer (, Section X); Treder (, pp. 30–43); Bergmann (, pp. 62–73); Straumann [334, 335]; Vizgin [384, 385]1; Bergia ; Goldstein and Ritter ; Straumann and O’Raifeartaigh ; Scholz , and Stachel . The section on Einstein’s unified field theories in Pais’ otherwise superb book presents the matter neither with the needed historical correctness nor with enough technical precision . A recent contribution of van Dongen, focussing on Einstein’s methodology, was also helpful . As will be seen, with regard to interpretations and conclusions, my views are different in some instances. In Einstein biographies, the subject of “unified field theories” – although keeping Einstein busy for the second half of his life – has been dealt with only in passing, e.g., in the book of Jordan , and in an unsatisfying way in excellent books by Fölsing  and by Hermann . This situation is understandable; for to describe a genius stubbornly clinging to a set of ideas, sterile for physics in comparison with quantum mechanics, over a period of more than 30 years, is not very rewarding. For the short biographical notes, various editions of J. C. Poggendorff’s Biographisch-Literarischem Handwörterbuch and internet sources have been used (in particular ). If not indicated otherwise, all non-English quotations have been translated by the author; the original text of quotations is given in footnotes.
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