  We discuss a spin glass reminiscent of the Random Energy Model, which allows
in particular to recast the Parisi minimization into a more classical Gibbs
variational principle, thereby shedding some light on the physical meaning of
the order parameter of the Parisi theory. As an application, we study the
impact of an extensive cavity field on Derrida's REM: Despite its simplicity,
this model displays some interesting features such as ultrametricity and chaos
in temperature.
