From The Economist, Dec 21st 2013
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.
The Ideas of Economists 