  We present a consistent picture of the respective role of density and
temperature in the viscous slowing down of glassforming liquids and polymers.
Specifically, based in part upon a new analysis of simulation and experimental
data on liquid ortho-terphenyl, we conclude that a zeroth-order description of
the approach to the glass transition should be formulated in terms of a
temperature-driven super-Arrhenius activated behavior rather than a
density-driven congestion or jamming phenomenon. The density plays a role at a
quantitative level, but its effect on the viscosity and the structural
relaxation time can be simply described via a single parameter, an effective
interaction energy that is characteristic of the high temperature liquid
regime; as a result, density does not affect the ``fragility'' of the
glassforming system.
