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.

Pauli writes about the conference in Geneva, saying that the mirror-problem remains unsolved, and that it seems clear that the masses of elementary particles are certain eigenvalues. He is disappointed with the results of Heisenberg's school. He needs help in the mathematics of these eigenvalue problems and wants to hear Källén's opinion on several points. He hopes that Källén's generation will produce constructive models of elementary particles.

Pauli adds an extract from a letter by Weisskopf about the Geneva conference . He has a positive attitude to Schwinger's last paper. He talks about a spinor-model by Gürsey where the neutrino and not the vacuum creates strangeness.

Pauli thanks Källén for his manuscript and letter. He makes a comparison of Källén's and Yang's results. He talks about results by Rohrlich for the case of charged scalar mesons in interaction with the quantized electromagnetic field. He also talks about results by Peasler and Kiroshita and about nu mber for charged vector mesons.

Pauli talks about identical results in papers by Thirring and by Hurst. He writes on the evaluation of a certain integral and on Peierl's method for the quantization of the form factor theory.