  For the past 40 years, Brueckner theory has proven to be a most powerful tool
to investigate systematically models for nuclear matter. I will give an
overview of the work done on nuclear matter theory, starting with the simplest
model and proceeding step by step to more sophisticated models by extending the
degrees of freedom and including relativity. The final results of a
comprehensive hadronic theory of nuclear matter are compared to the predictions
by currently fashionable two-nucleon force models. It turns out that a
two-nucleon force can, indeed, reproduce those results if the potential is
nonlocal, since nonlocality is an inherent quality of the more fundamental
fieldtheoretic approach. This nonlocality is crucial for creating sufficient
nuclear binding.
