A book cipher uses an ordinary published text as the shared secret. Sender and receiver own the same edition, and the encoded message is nothing more than a set of pointers into it. The appeal is obvious. No suspicious key material, no cipher machine, nothing incriminating on a shelf that also holds forty other books. For roughly four centuries this was one of the better options available to spies, conspirators and diplomats working without institutional support.

Walk the Egyptian rooms of any big museum and after a while you notice it. The noses are gone. Not every one, but enough of them that luck stops being a satisfying answer. Here is a seated official in dark stone, hands resting on his folded knees, heavy wig falling either side of a face that somebody worked over. The crowd moves past him without stopping. One woman has her phone out; she is photographing something else.

The building is a Renaissance palace on the Grand Canal, lit up at night, with a red banner hung over the balustrade advertising the games inside. You arrive by boat. There is a floating pontoon, an awning, a strip of red carpet leading to the door. Nothing about the approach suggests a business with a mathematical guarantee attached to it, which is exactly the point. The guarantee is real, though, and it is embarrassingly simple.

In 1870 the Prussian army was moving through eastern France and Lyon expected to be next. The archbishop, Jacques Ginoulhiac, made a public vow: if the city were spared, Lyon would build a church on the hill for the Virgin. The Prussians got as far as Nuits-Saint-Georges and stopped. Two years later the first stone went in, paid for by subscription, and the city spent the next twenty-four years honouring a promise made under panic.

Thucydides was an Athenian general and historian who lived in the fifth century BCE and wrote The History of the Peloponnesian War, the account of the twenty-seven-year conflict between Athens and Sparta that ended Athenian hegemony in the Greek world. He did not finish it. The narrative breaks off in 411 BCE, seven years before the war ended. What he left behind is nonetheless the foundational text of realist political thought and the first serious attempt to explain why states go to war.