Was Leonardo Da Vinci Machiavelli's informer? Tim Cornwell examines the evidence.

A new book by a political philosopher reads, in stretches, like a CIA hunt for a double-agent within its ranks. A paper trail places two men in the same places at the same time, working the same projects, sometimes even in the same building.

The period in this case, however, is Renaissance Italy. The suspects: Leonardo Da Vinci and Niccolo Machiavelli. Da Vinci and Machiavelli certainly co-existed in 16th-century Florence, but is it likely they knew each other?

In his book Machiavelli, Leonardo and the Science of Power, Roger Masters makes a strong circumstantial case not only that they met, but that Da Vinci was a serious influence on Machiavelli's thought.

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There is one major obstacle: Machiavelli, a prodigious memo-writer as a top Florentine bureaucrat, never mentions Leonardo - and Leonardo's notes scrupulously avoid Machiavelli. Here, it is hinted, there is an even more intriguing possibility, a spy story indeed.

Masters, professor of government at Dartmouth College, devotes most of his book to the thesis that Machiavelli's work, partly inspired by Leonardo, combined scientific and political analysis in a way that opened the window on modern thought.

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The book has met with a mixed reception in the United States: Masters' fellow political theorists warmed to the work, but historians seem more sceptical. Masters argues that Machiavelli learned the possibilities of technological innovation, and its limits, when he oversaw a failed attempt in 1503 to subdue Pisa by diverting the River Arno away from the city. Ten years before, Leonardo had sketched a plan for the peaceful diversion of the river. He was consulted by the Florentine government on the wartime project and reported his findings in person to a council at which Machiavelli must have been present.

It is the river project, Masters argues, that provides the fullest evidence that the two men were familiar; Machiavelli displays a keen interest in the kind of expertise with water flows that Leonardo possessed.

But Leonardo also left drawings of an armoured vehicle and a chariot armed with scythes. A "scythed chariot" turns up in Machiavelli's The Art of War. In 1504 he was one of two witnesses to a document settling payment for "The Battle of Anghiari", a painting commissioned from Leonardo.

But most mysterious of all is what Masters posits as the men's first meeting in Imola, Italy, in 1502. Machiavelli, at 37, was the senior envoy in a delegation to the court of Cesare Borgia. It was ten years before he wrote The Prince, after his arrest by the Medici.

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There was another Florentine in Imola: Leonardo Da Vinci, turning 50, pacing the streets as he measured out a detailed map of the city. Already famous as the painter of "The Last Supper", full of plans for weapons, Leonardo was hired by Borgia as "architect and general engineer ... charged with inspecting the places and fortifications of our states". Unlike Machiavelli, a loyal Florentine, Leonardo was a journeyman among the Italian city states, hiring out to employers in Milan, France and Rome.

Did the two men meet in Imola? Masters suggests it is almost impossible that they did not. Machiavelli's dispatches to Florence list many others, so why not Leonardo, a prominent and, from a military intelligence point of view, important figure? The answer: Leonardo was in fact Machiavelli's informant. On November 3, 1502, Machiavelli reports a conversation with an unnamed "first secretary" and later mentions a "friend" full of helpful information. There is no proof, but plenty of conjecture, that the friend was Leonardo.

Machiavelli was well aware his letters could be intercepted, and "knew that if Cesare knew that Leonardo was talking behind his back, Leonardo would get killed", says Masters. "Machiavelli's silences are as important as anything else. There are some cases,where the most important evidence is the dog that didn't bark."