The moon vehicle system was based on a modular design. One advantage of such a design – that failure in one component is less likely to compromise the whole – was demonstrated on the Apollo 13 mission. The astronauts were brought back safely to earth despite an explosion that damaged systems on the principal craft. Any engineer will tell you of the importance of making complex systems robust. You need inspections to prevent failure, to be sure: but since failures are inevitable it is equally important to try to ensure that the consequences of such failure are contained.

This observation is as relevant to economic and financial systems as to technological ones. Designing them with components too important to fail is a prelude to disaster, as we know. In the financial sector, the problem of disruptive linkages between components has become known as the problem of systemic risk – a term that is used in several different ways.

Often it describes macro-prudential risk, which arises mainly from the ability of financial market participants to persuade each other of absurd things. The cyclical booms and busts that result damage the non-financial economy, and economic policy can counteract this. Regulation should take away the punch-bowl as the party is starting, as William McChesney Martin, Federal Reserve chairman for nearly two decades until 1970, had it. But this is not a politically popular course. It is much easier, in the style of Alan Greenspan, to persuade yourself that what is plainly a party is just an outburst of rational exuberance.