Pauli sends forms to be filled out for CERN.

Pauli encloses a letter by Heisenberg and asks Källén to write to him.

Pauli expresses his doubts whether Källén's mathematics has anything to do with physics. He talks about Heisenberg's approach to the problem of field theory.

Pauli talks about the progress made by Taylor in Princeton and Bogoljubov in dispersion relations and he writes about the article for the Springer Handbuch.

Pauli talks more about Springer and time-dependent wave-packets in a Lee model with complex roots. He writes on the interpretation of isotopic spin.

Pauli replies to Källén's solution in his letter of the 9 January 1958 with additional remarks about $\lambda_{o}$-particles, the identity of "isospace" with Hilbert-space and the fourfold degeneracy of the vacuum and its removal.

Pauli writes that the crucial question for the spinor model is whether a reasonable formulation of an eigenvalue problem can be found for the masses of the particles. He talks about a special question concerning interaction in quantum mechanics: do products of field operators for the same point have a well defined meaning? He does not accept Källén's arguments against the spinor model.

Pauli comments on the publicity given to Heisenberg's work. He does not believe that any model will explain the symmetry diminution occuring at the transition from the isogroup to electromagnetic forces. preprint of his and Heisenberg's work.

Pauli replies to Källén's letter of the 18 March 1958. He is now sure that the explanation of strange particles having half-integer difference of spin and isospin by means of a degenerate vacuum is wrong. This changes his view on the one-spinor model with "Pauli-invariance", from which only particles with integer difference of spin and isospin can result (i.e. no$\lambda_{o}$-particles).

Pauli comments on Källén's paper. There are differences between his part I and part II. He talks about states with one electron-positron pair and about Dyson's view that only renormalized interactions existing in nature.