Pauli letter collection: letter to Gunnar Källén

Pauli writes on the non-physical ("pathological") solutions of quantum electrodynamics in case of strict validity of Lorentz-invariance. The mathematical consequences of this: "ghosts" and macroscopic acausalities and the postulate that for large photon- and quantumnumbers of electrons the classical theory must result as limiting case of quantum electrodynamics. One has to look if the equations of quantum electrodynamics have solutions which fulfil these three consequences.