An aeroplane belonging to the Swiss national airline, carrying 229 people, takes off on a scheduled flight from New York to Geneva. Fifty minutes out of Kennedy Airport, as the stewardesses roll their trolleys down the aisles of the McDonald Douglas MD-11, the captain reports smoke in the cockpit. Ten minutes later, the plane disappears off the radar. The gigantic machine, each of its wings 52 metres long, crashes into the placid seas off Halifax, Nova Scotia, killing all on board. Rescue workers speak of the difficulty of identifying what were, only hours before, humans with lives and plans. Briefcases are found floating in the sea.

1. If we do not dwell on the risk of sudden disaster and pay a price for our innocence, it is because reality comprises two cruelly confusing characteristics: on the one hand, continuity and reliability lasting across generations; on the other, unheralded cataclysms. We find ourselves divided between a plausible invitation to assume that tomorrow will be much like today and the possibility that we will meet with an appalling event after which nothing will ever be the same again. It is because we have such powerful incentives to neglect the latter that Seneca invoked a goddess.

2. She was to be found on the back of many Roman coins, holding a cornucopia in one hand and a rudder in the other. She was beautiful and usually wore a light tunic and a coy smile. Her name was Fortune. She had originated as a fertility goddess, the firstborn of Jupiter, and was honoured with a festival on the 25th of May and with temples throughout Italy, visited by the barren and farmers in search of rain. But gradually her remit had widened, she had become associated with money, advancement, love and health. The cornucopia was symbol of her power to bestow favours, the rudder a symbol of her more sinister power to change destinies. She could scatter gifts, then with terrifying speed shift the rudder's course, maintaining an imperturbable smile as she watched us choke to death on a fishbone or disappear in a landslide.

3. Because we are injured most by what we do not expect, and because we must expect everything ('There is nothing which Fortune does not dare'), we must, proposed Seneca, hold the possibility of disaster in mind at all times. No one should undertake a journey by car, or walk down the stairs, or say goodbye to a friend, without an awareness, w