  It is widely acknowledged that, for formal purposes, a minimally coupled
scalar field is equivalent to an effective perfect fluid with equation of state
determined by the scalar potential. This correspondence is not complete because
the Lagrangian densities P and -rho, which are equivalent for a perfect fluid,
are not equivalent for a minimally coupled scalar field. The exchange between
these two Lagrangian densities amounts to exchanging a canonical scalar field
with a phantom scalar field.
