  The effects of minimally coupling a gravity to matter on a flat
Robertson-Walker geometry are explored. Particular attention is paid to the
evolution of the symplectic structure and the Liouville measure it defines. We
show that the rescaling freedom introduced by choice of fiducial cell leads to
a symmetry between dynamical trajectories, which together with the Liouville
measure provides a natural volume weighting explanation for the generic
existence of attractors.
