Pauli is ready to contribute to the Festschrift for Sommerfeld. In connection with experiments in the U.S.A. on the shift of S-terms in the H-atom he has returned to quantum electrodynamics.

Pauli criticizes certain passages in Heisenberg's manuscript of the Dialectica-article, and asks to correct them. He writes about the "fundamental length" in quantum theory. The problem is not how to introduce such a length, but how to get rid of the multitude of lengths (Compton-length for each particle) in the present theory.

Pauli cannot come to Geneva, but would like to hear Heisenberg's opinion on non-local interaction and hear about biological literature.

Pauli asks about quantized field theory. He makes some remarks on Heisenberg's article for the Bohr-Festschrift, and on Einstein idea of physical reality: "the isolated observer". More about "incomplete descriptions" in physics.

Pauli talks about a paper by Kita who found an error in Heisenberg's calculations. Pauli doubts that Heisenberg's S-matrix can remain unitary.

Pauli replies to Heisenberg's letter of 10 December. He writes on clarifications of the differences in their respective views on the necessary presuppositions for a quantized field theory. He has a short discussion of quantum-electrodynamics and the Lee model.

Pauli talks on further calculations about dipole-ghosts. He asks about Eta-matrix.

Pauli talks about analysis of the defects of Heisenberg's approach and of some errors in his conclusions.

Pauli replies to Heisenberg's letter of 20 February.