  We show that essentially pure classical thermodynamics is sufficient to
determine Bekenstein's formula for the black hole's entropy, $S=\eta A$. We
base our reasoning on the minimal assumption that since black body radiation is
describable by classical thermodynamics, so is the complete black hole-Hawking
radiation system. Furthermore, we argue that any non-linear correction to the
black hole entropy must be quantum mechanical in nature. The proportionality
coefficient, $\eta=1/4\ell_P^2$, must be calculated within a semi-classical or
full-fledged quantum mechanical framework.
