It is observed that unlike the versions which rely on auxiliary potentials designed to cancel diagrams exactly, the continuous version of the Brueckner-Bethe-Goldstone theory can be used to calculate the properties and in particular the ground state energy of repulsive many body systems. The only proviso amounts to a trivial requirement on the effective mass at the Fermi momentum. These results are related to those concerning the predictions of the corresponding versions of Green's function theory for the analytical properties of the mass operator. The case of attractive many body systems is also considered.