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.

Pauli wrote to Kofoed-Hansen to express some disageement about a paper on neutrino reactions.

Mercier has prepared his contribution to the symposium of the Zürich congress.

Källén talks about calculations as regards the non-unitarity of the S-matrix in the Lee model.

Källén writes on certain mathematical problems of the Lee model.

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.

Källén attempts to clarify for himself the contents of the papers by Landau and his collaborators.