We develop an approach to quantum dynamics based on quantum phase space
trajectories. The latter are built from a unitary irreducible representation of
the symmetry group of the respective classical phase space. We use a quantum
action functional to derive the basic equations. In principle, our formulation
is equivalent to the Hilbert space formulation. However, the former allows for
consistent truncations to reduced phase spaces in which approximate quantum
dynamics can be derived. We believe that our approach can be very useful in the
domain of quantum cosmology and therefore, we use the cosmological phase space
example to establish the basic equations of this formalism.Comment: 11 pages, 4 figures, the new version contains improved discussion