One of history’s more curious encounters occurred in early March 1766 at a country estate in southern England, near Dorking. The estate belonged to Daniel Malthus, a gentleman of independent means and wide intellectual interests. The philosophers David Hume and Jean-Jacques Rousseau were traveling in the neighborhood, seeking a house for Rousseau, who had recently arrived in England under Hume’s patronage after being driven out of Switzerland.

Daniel Malthus was known to both philosophers, at least by correspondence, so they paid him a brief visit, in the course of which they saw his son Thomas, then just three weeks old. So there, presumably in the same room, were Hume, Rousseau, and the infant Thomas Malthus. It was an odd grouping: the serene empiricist, the neurotic social optimist, and the future oracle of demographic doom.

Hume had actually dabbled in demography himself some years earlier. He had been...

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