Pauli thanks Einstein for the proofs of the paper he has written with Mayer and he expreses the greatest doubts as to the physical valididity of its basic idea, to describe two particles with different masses by a wave-function in an ordinary space-time continuum.

Pauli emphasizes that in spite of the difficulties his attitude to the theory of holes is not negative. He gives some comments on the magnetic moment of nuclei and of the neutron. If the laws of nature are completely symmetrical with regard to positive and negative electricity, a negative proton should also exist and the free neutron should exist in two states, if its magnetic moment is different from zero.

Pauli reacts in a very positive way on Heisenberg's letter of the 6 November 1933 and hopes he will continue his calculations.

Pauli proves that Heisenberg's theory given in his letter of the 6 November 1933 is identical with a former theory developed by Pauli and Heisenberg.

Pauli agrees with Heisenberg's pessimism regarding infinite energies, but he thinks one should try to separate the problem of self-energy from that of the hole theory.

Pauli does not believe that the theory of holes can be improved without essentially new ideas.

Pauli writes of Weisskopf's and his own result that the material energy of photons becomes infinite in their theory. He remarks about "substraction physics": the fact that polarizability of the vacuum and self-energy of material particles are both becoming infinite must have a common root. The real progress of the theories will be found in the amalgamation of electromagnetic field and matter field.

Addition to the last letter of the 24 October 1934 (heisenberg_0205).

Pauli is interested in Heisenberg's new calculations on the hole theory and on the problem of magnetic nuclear moments.

Pauli believes that the introduction of singulary density matrices is not the physically correct way to a solution.