  The brick-wall model seeks to explain the Bekenstein-Hawking entropy as a
wall-contribution to the thermal energy of ambient quantum fields raised to the
Hawking temperature. Reservations have been expressed concerning the
self-consistency of this model. For example, it predicts large thermal energy
densities near the wall, producing a substantial mass-correction and,
presumably, a large gravitational back-reaction. We re-examine this model and
conclude that these reservations are unfounded once the ground state---the
Boulware state---is correctly identified. We argue that the brick-wall model
and the Gibbons-Hawking instanton (which ascribes a topological origin to the
Bekenstein-Hawking entropy) are mutually exclusive, alternative descriptions
(complementary in the sense of Bohr) of the same physics.
