Pauli writes about the isotopic-spin problem.

Pauli continues his last letter of the 13 December.

Heisenberg replies to Pauli's letter. They agree about dipole ghosts. The eigenvalue spectrum has to follow from the theory, if this is correct. He talks about his equation which he regards as the correct equation of matter from which all elementary particles can be derived.

Heisenberg suggests an alternative for his operator O which was criticized by Pauli and develops some results from this new start.

Pauli talks about the problem of mathematical formulation of symmetry-violations in the case of the dichotomy connected with the interpretation of dipole ghosts and complex ghosts. He hopes for progress in the question of leptons. There is a more positive attitude on the question of indefinite metric.

Pauli talks more about Springer and time-dependent wave-packets in a Lee model with complex roots. He writes on the interpretation of isotopic spin.

Pauli writes on further calculations about the physics of elementary particles, and about more group theoretical considerations. Possibly there exists complementary between the space-time description in the micro-world and the physical probability.

Pauli is very optimistic about the results so far. He suggests as a title for the the joint paper: "On the interpretation of isotopic spin". He gives some moreconclusions. He makes some mathematical comments on unitary and pseudo-unitary transformations, and the dichotomy of the vacuum.