In this letter, Pauli makes some remarks on Jordan's cosmology and the ergodic theorem in quantum mechanics.

Pauli writes about his impending visit to Princeton. He talks about papers by Dyson and Marshall.

Pauli makes comments on Edmonds' lecture notes and on Glaser's work on the Lee model. He writes of a possible sign error in Heisenberg's S-matrix. He writes about his possible trip to Princeton.

Heisenberg talks about the arguments of the Japanese physicist Kita. He is against Pauli's rejection in principle of infinite metrics in the Hilbert-space. He writes on his own optimism as regards the present state of field physics.

Pauli says that he has not recently produced anything new in physics, partly because of many deaths in his environment in Zurich, including Placzek, Pauli's father, the wife of Carl G Jung and Hermann Weyl victim of a heart attack. He also speaks about his latest trip to Hamburg, where he saw Jordan, the astronomer Heckman and his former "chief" Lenz. Lenz will retire next spring, and there were some discussions about his succession. He talks to Bohr about the project of a Scandinavian Institute of Physics in Copenhagen. Pauli has also received another political letter from Bernadotte regarding the "Mainau statement" and asks Bohr for his opinion.

Pauli replies to Heisenberg's letter of 25 December, especially about field-operators. He writes about his paper for the forthcoming meeting in New York on quantum electrodynamics.