  We analyze the high temperature (or classical) limit of the Casimir effect. A
useful quantity which arises naturally in our discussion is the ``relative
Casimir energy", which we define for a configuration of disjoint conducting
boundaries of arbitrary shapes, as the difference of Casimir energies between
the given configuration and a configuration with the same boundaries infinitely
far apart. Using path integration techniques, we show that the relative Casimir
energy vanishes exponentially fast in temperature. This is consistent with a
simple physical argument based on Kirchhoff's law. As a result the ``relative
Casimir entropy", which we define in an obviously analogous manner, tends, in
the classical limit, to a finite asymptotic value which depends only on the
geometry of the boundaries. Thus the Casimir force between disjoint pieces of
the boundary, in the classical limit, is entropy driven and is governed by a
dimensionless number characterizing the geometry of the cavity. Contributions
to the Casimir thermodynamical quantities due to each individual connected
component of the boundary exhibit logarithmic deviations in temperature from
the behavior just described. These logarithmic deviations seem to arise due to
our difficulty to separate the Casimir energy (and the other thermodynamical
quantities) from the ``electromagnetic'' self-energy of each of the connected
components of the boundary in a well defined manner. Our approach to the
Casimir effect is not to impose sharp boundary conditions on the fluctuating
field, but rather take into consideration its interaction with the plasma of
``charge carriers'' in the boundary, with the plasma frequency playing the role
of a physical UV cutoff. This also allows us to analyze deviations from a
perfect conductor behavior.
