Pauli writes on a question about the S-matrix in Lee's model. "Is it unitary?"

Pauli talks about Felix Bloch's interest in Schafroth's work on the ideal Bose-gas. He talks about his study on Lee's paper (in: Phys. Rev. : 95 (1954), pp.1329).

Pauli wants to write first to Lee before the proposed paper by Pauli and Källén is written. He writes on a definition of a "ghost state". He expresses doubts about the Tamm-Dancoff approximation in the case of more general systems. He makes some mathematical remarks.

Pauli writes on the difficulties of quantum electrodynamics and the various attempts to overcome them. Pauli expresses doubts about the renormalization method.

Pauli thanks Källén for the manuscript. He writes about the Tamm-Dancoff method and some known negative attitude towards it. Small changes in the manuscript and the re-writing of the appendix.

Pauli asks for classification of Klein's work.

Pauli has some more suggestions for minor changes in the paper.

Pauli criticizes Klein's notation and a commutator relation. He refers him to Rosenfeld (1930).

Pauli talks about his completely different views from those of Schrödinger on the interpretation, on the difficulties and the tendencies of quantum mechanics. He makes a formulation of a few theses about physical theories. He says that among the physicists opposed to quantum mechanics Einstein is understood best by him.

Pauli hopes to meet Heisenberg in Bern.