Pauli does not believe that there exists a key for derivation of a quantum S-matrix from an arbitrary classical Hamiltonian. Without a theory which determines e2/(n.c) no real progress is possible.

Pauli asks Heisenberg for a contribution to a special issue of "Dialectica" about complementarity.

Pauli replies to some points in Heisenberg's letters (18th April 1947 and 20th June 1947). The idea of an analytical continuation of the S-matrix is a failure. He is scepticalabout future theories, especially as the problem of the dimensionless constant e2/(n.c) is not solved. He doesn't believe in the existence of a universal length. He does not believe either in the mathematical existence of a convergent quantum electrodynamics with arbitrary e2/(n.c). He writes of his impending visit to Sweden and Copenhagen.

Pauli is glad that Heisenberg will write an article for Dialectica, and that he has been able to visit England. He writes him to read a paper by Lewis, a pupil of Oppenheimer in: Phys. Rev., 15 January 1948.

Pauli thanks Heisenberg for the corrections to the Dialectica article. No formalism which takes into account only protons and electrons can be final. He and Heisenberg agree that the masses of elementary particles are not fundamental constants that they will result as eigenvalues from a future theory. But the concept of "universal length" does not seem useful to Pauli.

Pauli criticises a paper by three pupils of Heisenberg on "the formulation of quantized field theories". He makes some remarks on Lee's model.

Pauli gives more calculations and questions about Heisenberg's formalism.

Pauli comments on a letter by Källén and his problem of two N-particles.