Fierz makes some additions to his letter of the 15th May 1951.

Fierz questions Szilard's assertion on reversibility of mixture of systems. He considers how measurement might be seen as turning pure state into a mixture.

Fierz gives detailed comments on two papers by van Hove.

Einstein replies to Pauli's criticism of his new field theory, the "Fernparallelismus". He thinks that it is too early to dismiss it in the way Pauli does. Einstein is not against the quantum theory but he does not believe that real insight can be reached through statistical laws. He asks Pauli to look again in his theory without any prejudice.

Ehrenfest is looking forward to see Pauli in Leiden. he mentions some papers by Heisenberg, Hund and on group theory, on which he wants to ask Pauli several questions.

Ehrenfest sends a short note in which he expresses his doubts as to the separability of the operator-function M representing the spin interaction of the two He-electron.

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.

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 is on vacation in Arosa with Harald. He hopes to see Pauli when coming through Hamburg on their return and will talk about Pauli's planned visit to Copenhagen. Bohr has sent an entirely new manuscript to Berlin for "Naturwissenschaften : 16 (1928), pp.245".

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.