'The rise and metamorphosis of double-entry bookkeeping is one of history's best-kept secrets and most important untold tales ...Through its logic we have let the planet go to ruin-and through its logic we now have a chance to avert that ruin.' Our world is governed by the numbers generated by the accounts of nations and corporations. We depend on these numbers to direct our governments, organisations, economies, societies. But where did they come from-and how did they become so powerful? The answer to these questions begins in the Dark Ages, with the emergence in northern Italy of a new form of accounting called double-entry bookkeeping. The story of double entry reaches from the Crusades through the Renaissance to the factories of industrial Britain and the policymakers of the Great Depression and the Second World War. At its heart stands a Renaissance monk, mathematician and magician, and his celebrated treatise for merchants. With double entry came the wealth and cultural efflorescence that was the Renaissance, a new scientific worldview, and a new economic system: capitalism. Over the past one hundred years accounting has flourished to an astonishing degree, despite the many scandals it has left in its wake. The figures double entry generates have become a sophisticated system of numbers which in the twenty-first century rules the global economy, manipulated by governments, financial institutions and the quant nerds of Wall Street. And the story of double entry is still unfolding-because today it might be our last hope for life on earth.