  The second half of this essay tells the story of the genesis and early
development of the notion of black hole entropy, in the style of an after
dinner talk (apart from a long technical footnote). The first half sketches the
development of the concept of entropy, beginning with Carnot, passing via
Thomson, Clausius, Boltzmann, Planck, von Neumann, and Shannon on the way to
Bekenstein, and the essay ends with a retrospective on Lema\^itre. The central
theme is the fault-tolerant way that profound insights have emerged from simple
(yet subtle) thermodynamic reasoning.
