Reprinted in The Physical Review - the First Hundred Years, AIP Press (1995) CD-ROM.
The various types of exchange forces that are being used in current discussions of nuclear structure may all be simply expressed in terms of a formalism which attributes five coordinates to each "heavy'' particle and applies the Pauli exclusion principle to all the particles in the system. The simplest assumption for the interaction law is that which implies equality of proton-proton and neutron-neutron forces and also equality with the proton-neutron forces of corresponding symmetry. This is in
accord with the empirical knowledge of these interactions at present.
Related references See also E. Fermi, Z. Phys. 88 (1934) 161;
W. Heisenberg, Z. Phys. 77 (1932) 1;
Proposal of the isotopic spin (for the nucleon).