  We present a new formalism to study large-scale structure in the universe.
The result is a hierarchy (which we call the "Helmholtz Hierarchy") of
equations describing the phase space statistics of cold dark matter (CDM). The
hierarchy features a physical ordering parameter which interpolates between the
Zel'dovich approximation and fully-fledged gravitational interactions. The
results incorporate the effects of stream crossing. We show that the Helmholtz
hierarchy is self-consistent and obeys causality to all orders. We present an
interpretation of the hierarchy in terms of effective particle trajectories.
