  Apparently, all partial differential equations that describe physical
phenomena in space-time can be cast into a universal quasilinear, first-order
form. In this paper, we do two things. First, we describe some broad features
of systems of differential equations so formulated. Examples of such features
include hyperbolicity of the equations, constraints and their roles (e.g., in
connection with the initial-value formulation), how diffeomorphism freedom is
manifest, and how interactions between systems arise and operate. Second, we
give a number of examples that illustrate how the equations for physical
systems are cast into this form. These examples suggest that the first-order,
quasilinear form for a system is often not only the simplest mathematically,
but also the most transparent physically.
