How is it that history makes an unexpected leap forward?
Take the sudden onset of the economic crisis. We were told this could never happen again, but banks are failing, the financial system is in turmoil and a recession is opening up beneath our feet.
The recession is hardly the only example in recent years of a sharp disruption to the flow of events. The 9/11 attacks and their consequences were utterly unforeseen, and marked a turning point after which many important things in the world were never the same again.