  There is a widespread delusion that in theoretical nuclear physics protons
and neutrons are the real thing, and nucleons are not more than a
mathematically equivalent formality. It is shown that, on the contrary,
nucleons are the real thing, because only a part of the theory is essentially
identical to proton-and-neutron theory, whereas the remaining part is
physically relevant. The approach is general. Thus, this is a paradigm of
relation of a wider and a more narrow theory, so that the wider theory
describes reality better. Also the relation of disjoint domains to the
exclusion principle is clarified. A general fermion theory of how to
distinguish identical particles is presented.
