Pauli writes about longitudinal photons. The principle of "renormalization" of charge and mass works satisfactorily only in special interactions. Another question: will it be necessary to modify the space-time concept for small dimensions?

Pauli is looking forward to reading Jauch's forthcoming book on electrodynamics. He would prefer not to read the book in the proof-state, for this means a certain responsibility.

Pauli writes on the concept of chance in biology and on the lack of a quantitative definition of biological probabilities. He suspects that the present conception of evolution is still essentially incomplete. The two parameters of selection and mutation are logically independent of each other. It is not the gene that is thesmallest unit of life but the cell as Bohr has emphasized.

Pauli makes further critical remarks about Jordan's cosmology. He writes that some of his assumption should be derived mathematically from a new nuclear theory.

Pauli writes about Jordan's article in the Bohrfestchrift which deals with the role of chance in biology. Pauli criticizes the use of the concept of chance in biology and evolution in particular.

Pauli writes about the degrees of simplicity of field quantization theories of Heisenberg, Schwinger and Feynman-Dyson. He is interested, at present, in which interactions become convergent after renormalization and which do not. He speaks of results obtained by Matthews (Cambridge) for certain meson theories.

Pauli thanks Källén for his manuscript and letter. He makes a comparison of Källén's and Yang's results. He talks about results by Rohrlich for the case of charged scalar mesons in interaction with the quantized electromagnetic field. He also talks about results by Peasler and Kiroshita and about nu mber for charged vector mesons.

The letter talks of progress in meson theory.

Pauli refers Klein to Belinfante (Physica) for side conditions needed for Klein's equations. Pauli describes the conditions.

Pauli talks about his interest in the question for which interactions the Schwinger-Dyson formalism of renormalization leads to a succesful result. He comments on Kronig's story of his dream about Ehrenfest's sermon on love. He thinks that the roots of feeling are as deep as those of thought. He says that love is a force of nature. He says that in the period of the renaissance there was not yet a separation between cognition and inner experience, and at that time love was the most important subject of philosophy (e.g. Ficino and Pico de la Mirandola and Leone Ebreo).