  I give a fairly systematic and thorough presentation of the case for
regarding black holes as thermodynamic systems in the fullest sense, aimed at
students and non-specialists and not presuming advanced knowledge of quantum
gravity. I pay particular attention to (i) the availability in classical black
hole thermodynamics of a well-defined notion of adiabatic intervention; (ii)
the power of the membrane paradigm to make black hole thermodynamics precise
and to extend it to local-equilibrium contexts; (iii) the central role of
Hawking radiation in permitting black holes to be in thermal contact with one
another; (iv) the wide range of routes by which Hawking radiation can be
derived and its back-reaction on the black hole calculated; (v) the
interpretation of Hawking radiation close to the black hole as a
gravitationally bound thermal atmosphere. In an appendix I discuss recent
criticisms of black hole thermodynamics by Dougherty and Callender. This paper
confines its attention to the thermodynamics of black holes; a sequel will
consider their statistical mechanics.
