  We reinvestigate the conditions for stable matter solutions in the
Nambu--Jona-Lasinio (NJL) model. In mean field approximation the NJL model can
be regarded as an extension of the Walecka mean field model to include negative
energy fermion states. While this extension is necessary to allow for a chiral
phase transition, it was found some time ago that at the same time it destroys
the wanted saturation properties of the Walecka model. We reformulate this
problem in terms of the thermodynamic potential and find that there is indeed a
connection between these two features. We show that the minimum of the
thermodynamic potential which corresponds to stable nuclear matter in the
Walecka model is shifted from a finite to zero effective fermion mass in the
chiral NJL model. This shift is closely related to the chiral phase transition.
Under certain conditions the shifted minima may still lead to stable matter
solutions but only in the chirally restored phase. We discuss a possible
interpretation of these solutions as a schematic bag model description.
