Entropy from Carnot to Bekenstein

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.