Bohr was glad to receive Pauli's article which gives an excellent account of Einstein's pioneering work in quantum theory. Bohr does not know what to answer to Pauli's questions concerning some changes Bohr should make in his paper. The detailed account of the discussions which he had first planned turns out to be much too long for the occasion. Therefore, he is trying to confine himself to the general epistemological aspects of the attitude taken in the discussions, and he has started on a much shorter article which he hopes to have ready very soon. In a few days Bohr will write to Pauli again and tell him how things stand so that it can be decided how to make the best out of it all.

Bohr thanks Pauli for his congratulations on his 65th birthday. He writes about the double role of a human being as an observer and as an actor in the theatre of life. He also speaks of the preparation of an article on free will and life, and of a book based on his Grifford lecture.

Professor Bohr was very glad to have Pauli's letter of the 26th March. Since Bohr wrote last he had a visit from Delbrück, who, as Pauli knows, will stay in Göttingen this summer. This visit gave an opportunity for many instructive and interesting discussions with Danish biochemists and biologists. Bohr feels still more convinced that the general considerations about organic development, about which Bohr wrote to Pauli in January, hit a quite important point. He hopes to be able to give a more clear account of such questions in a lecture "On the unity of knowledge" at "the bicentennial celebrations" at the Columbia University in Autumn.

Bohr's reply to Pauli's letter of the 15th February. He explains his use of the phrase "detached observer" and writes on the necessity to clarify the terminology.

Bohr writes on the significance of detachment in scientific work, the interaction of the observer with the course of events, the freedom of choice of the experimental arrangement and on the difference between the role of observer in biology and quantum mechanics.

Ever since Pauli and his wife gave the Bohrs the great pleasure of coming to Bohr's birthday, it has been on Bohr's mind to write to Pauli to say how much their friendship has meant to him during the many years, and how thankful he was again to feel how well they understand each other. Not least when one has come so far in years and has to seek the greatest joy in following the younger generations' progress, all the kindness shown to him was a very great encouragement. It was sad to hear about the death of Pauli's elderly father, and they are all sorry for the loss of Placzek and Weyl. Under these conditions Pauli's and his wife's visit to Princeton will be a great encouragement to all of them, not least for Oppenheimer, of whom they are often thinking. Bohr mentions some questions about the invitation to Lindau, he says that he and his wife plan to go there in the summer. It should be a special joy if Pauli and his wife also came.

Bohr does not agree with Pauli's and Heisenberg's criticism of Thomas' work in: Nature : 117 (1926), pp. 514. He discusses the problem in detail and asks for further comments from Pauli.

Bohr will pass through Hamburg together with Klein on their way to Holland. He hopes that Pauli will join him on his way back to Copenhagen.

Bohr has been thinking of writing to Pauli since the appearance of the Sommerfeld Festschrift, but the letter grew in bulk into a paper, which is not yet finished. Klein is working on relativity problems and is very hopeful about his work for: Z. Phys. : 53 (1928), pp.157".

Bohr thanks Pauli for his letter which gave him a welcome occasion for a small heart-outpouring. He has recently returned to his old primitive view on the problems of the electron theory after having been alarmed by Pauli's difficulty with the infinite self-energy of the electrons.