The search for a satisfactory nuclear potential in Quantum
Field Theory (hereafter referred to as Q.F.T.) has been long and
the attempts have been numerous. The difficulty of the problem
is twofold: firstly, because a potential is not Lorentz covariant,
we have to find a way to convert the relativistic formalism of
Q.F.T. into its non- relativistic equivalent; secondly, because no
satisfactory method of calculating quantities appearing in Q.F.T.
has yet evolved, we have to find ways to approximately calculate
the potential that emerges out of the dissolution of the first
difficulty. Different authors took different attitudes to either
of these problems, and the result was a proliferation of proposals
with varying degrees of success. Diverse though these proposals
are, they fall into four main classes.