Pauli continues with his calculations with hyper-complex numbers. He adds some remarks on "neutretto"-scattering and the theory of mesons.

A short addendum to the last letter.

Pauli writes about some free time he spent with A. Thellung, and about his (Pauli's) relationship with the Kramers.

Pauli agrees with Källén about an error in Thirring's paper. The question now is whether Hurst's paper is correct.

Pauli talks about questions in connection with the preparation of a lecture on problems of quantum electrodynamics: 1) about Lee's non-relativistic models for renormalization, 2) about longitudinal photons, 3) about a paper by Green published in: Physical Review 95, 548 - 1954.

Pauli thanks Källén for his reply. He objects to his views on the gauge-invariance. He writes on the danger for a theoretical physicist to stick to one special formalism. This is also Källén's mistake in his formulation of the Lee-model.

Pauli replies to Källen's letter and continues the topics in Pauli's last letter. The coupling-free case has to be eliminated from Lee's model because it does not exist in nature.

Pauli sends a list of people to whom preprints of their paper should be sent. He comments the reaction of Heisenberg and his pupils to the paper.

Pauli writes on the non-physical ("pathological") solutions of quantum electrodynamics in case of strict validity of Lorentz-invariance. The mathematical consequences of this: "ghosts" and macroscopic acausalities and the postulate that for large photon- and quantumnumbers of electrons the classical theory must result as limiting case of quantum electrodynamics. One has to look if the equations of quantum electrodynamics have solutions which fulfil these three consequences.