Heisenberg remarks on a probable mistake in Pauli's letter.

Pauli explains what he means by "antisymmetric metric". Hereby one gets the masses of leptons different from zero.

Pauli writes about his impending trip to the USA. He writes about his work with Heisenberg.

Heisenberg asks for clarification of certain of Pauli's arguments.

Pauli is glad about Heisenberg's impending visit. He writes a few words about the next stage of the discussion, especially the eigenvalue problem for rest masses.

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 says that he reached similar conclusions to Heisenberg.

Letter amplifying his cable from the boat to the USA, urging Heisenberg not to publish now and not to send anything off before giving Pauli an opportunity to express his view. He wants Heisenberg's clear reply on the degeneration of the vacuum, mirror states, the metric in Hilbert space and the eigenvalue problem. He gives more details about the last point. He writes about reactions of colleagues (also Bohr) in New York to their ideas. The scepticism of Yang and Lee and of Heisenberg's pupils, has made Pauli feel that publication should be deferred.

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.