Pauli letter collection: letter to Wolfgang Pauli

Heisenberg replies to Pauli's questions on the possibility of calculating the total energy of matter created by the introduction of fields. He gives suggestions for the improvement of the paper and talks about Pauli's theory regarding the Klein-Gordon equation and the possibility of finding by this a method of field quantization, provided that modifications are introduced in the quantization rules of field, leading to spinors and Fermi-statistics. He writes about the scattering of light by light.