  In the hadrodynamics (QHD) there are two methods to take account of the
contribution of negative-energy states in the relativistic random phase
approximation (RRPA). Dawson and Furnstahl made the ansatz that the Dirac sea
were empty, while according to the Dirac hole theory the sea should be fully
occupied. The two methods seem contradictory. Their close relationship and
compatibility are explored and in particular the question of the ground-state
(GS) instability resulting from Dawson-Furnstanhl's ansatz is discussed.
