Pauli letter collection: letter to Gunnar Källén

Pauli replies to Källén's letter of the 18 March 1958. He is now sure that the explanation of strange particles having half-integer difference of spin and isospin by means of a degenerate vacuum is wrong. This changes his view on the one-spinor model with "Pauli-invariance", from which only particles with integer difference of spin and isospin can result (i.e. no$\lambda_{o}$-particles).