  Starting from full quantum field theory, various mean field approaches are
derived systematically. With a full consideration of external source
dependence, the stationary phase approximation of an action gives a nuclear
mean field theory which includes quantum correlation effects (such as
particle-hole or ladder diagram) in a simpler way than the
Brueckner-Hartree-Fock approach. Implementing further approximation, the result
can be reduced to Hartree-Fock or Hartree approximation. The role of the source
dependence in a mean field theory is examined.
