WILLIAM PETTY was an innovation machine. He designed an early form of catamaran, conceived of a mechanical grain planter, proposed attaching engines to boats and patented a “double-writing” instrument (it produced an extra copy of whatever a writer put down on paper). Petty, who died at Christmas in 1687, was also an innovator in the world of theories. By tinkering with data and simple models, this little-known Englishman came up with many of the ideas—how to measure GDP, why the money supply and banks matter, how lasting unemployment affects the economy—that form the bedrock of modern economics.
Born in 1623, the young Petty showed an early interest in clockmaking and joinery but did not enter either trade. His hometown, Romsey, is close to the sea and he left home on a ship, as a cabin boy, aged 14. That brought the first of two mishaps that changed the course of his life. After breaking his leg in a nautical accident he was put ashore for treatment in Normandy. As the injury healed he enrolled in a Jesuit college in Caen and developed an interest in anatomy, which he went on to study in Amsterdam and Paris. In Paris he met and worked for Thomas Hobbes, whose empiricism was a deep influence. Collecting data and making real-world observations became central to his work.
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