Pauli makes some comments on Källén's exposition on "Retarded functions of two variables".

Pauli replies to Källén's letter of the 3 October 1956. He writes about a proof by Symanzik concerning dispersion relations.

Pauli talks about a letter he received from Heisenberg about his recent considerations regarding the Lee model. The problem is where does the unitarity of the S-matrix fail in the case of indefinite metric.

Pauli makes some criticisms of some of Källén's results "not in order to criticize but in order to learn".

Pauli replies to Källén's letter of the 2 January 1957. He agrees to his interpretation of Heisenberg's ideas. He talks about Heisenberg's tendency to modify his assumptions if he is refuted.

Pauli replies to Källén's letter of the 22 and the 28 January 1957. Pauli declares himself uninterested at the moment in indefinite metric. He talks about parity and the two-component theory of the neutrino. He writes on the possible existence of symmetries and the theorem of "strong reflection" and its consequences, on the possible influence of a yet unknown field and arguments from experience against this hypothesis and on connections of parity-violation with other not yet explained facts. He wonders why strong interactions have left-right symmetry. The present formulation of the theory allows for too many posibilities. Posssibly interactions create space, time and their symmetries.

Pauli talks of technicalities about a visit to Copenhagen in spring. Heisenberg agrees with Källén's results and yet he wants to retain the dipole ghosts. He writes about experimental results with oriented nuclear spin in Co58 by Mrs. Wu.

Pauli asks Källén to tell Gustafson that he cannot come to Sweden before his trip to Berkeley. He talks about Jost's example which showed that Källén representation's by retarded functions cannot be derived. He also writes on Thirring's model.

Pauli talks about Springer concerning the Handbuch article. He writes more on Thirring's model.

Pauli gives the result of his thoughts about Heisenberg's Lee model and sends a detailed discussion on infinite metric and complex energy values. He writes on possible generalization of Heisenberg's formalism.