  Starting with a formally exact diagrammatic kinetic theory for the
equilibrium correlation functions of particle density and current fluctuations
for a monatomic liquid, we develop a theory for high density liquids whose
interatomic potential has a strongly repulsive short ranged part. We assume
that interparticle collisions via this short ranged part of the potential are
sufficient to randomize the velocities of the particles on a very small time
scale compared with the fundamental time scale defined as the particle diameter
divided by the mean thermal velocity. When this is the case, the graphical
theory suggests that both the particle current correlation functions and the
memory function of the particle density correlation function evolve on two
distinct time scales, the very short time scale just mentioned and another that
is much longer than the fundamental time scale. The diagrams that describe the
motion on each of these time scales are identified. When the two time scales
are very different, a dramatic simplification of the diagrammatic theory at
long times takes place. We identify an irreducible memory function and a more
basic function, which we call the irreducible memory kernel. This latter
function evolves on the longer time scale only and determines the time
dependence of the density and current correlation functions of interest at long
times. In the following paper, a simple one-loop approximation for the
irreducible memory kernel is used to calculate correlation functions for a
Lennard-Jones fluid at high density and a variety of temperatures.
