Pauli congratulates Heisenberg on the birth of his child. He makes a remark on the cut-off frequency of long waves and about Yukawa's theory of nuclear forces.

Pauli talks more about cosmic rays and the occurence of light particles (electrons) in showers.

Pauli reacts sharply on "non-scientific secondary conditions" of certain publishers as regards the racial origin of scientific authors. He talks further about cosmic ray experiments and their interpretation.

Pauli gives a synopsis of his planned paper at the Solvay congress. He makes more comments on Heisenberg's damping-terms in the scattering of mesons by protons.

Pauli writes of the decision to postpone the congress because of the political situation.

Pauli has read with great interest Heisenberg's paper on the S-matrix, but as no method is given how to determine it, the work remains purely programatic.

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 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.