  With an appropriate choice of parameters, a higher derivative theory of
gravity can describe a normal massive sector and a ghost massless sector. We
show that, when defined on an asymptotically de Sitter spacetime with Dirichlet
boundary conditions, such a higher derivative gravity can provide a framework
for a unitary theory of massive gravity in four spacetime dimensions. The
resulting theory is free not only of higher derivative ghosts but also of the
Boulware-Deser mode.
