In the history of humankind, there has rarely been a disaster like the New Orleans flood without a theodicy to go along with it. The word "theodicy," coined in the 18th century by the philosopher Gottfried Leibniz, derives from Greek roots invoking the "justice of the gods." A theodicy is an attempt to show that such justice exists, to prove that we really do live in what Leibniz insisted was the "best of all possible worlds."

So theodicies have been plentiful after earthquakes, floods and droughts. Explanations are readily offered: disasters are the wages of sin, they herald an apocalyptic age, they cleanse the earth of evil. Theodicies aim to demonstrate that devastation does not really disrupt or overturn our understanding of the moral and social order. Instead, disorder provides evidence of order. The theodicy is that order. It explains forces that seem to lie beyond human powers, evils that lie beyond human cause.

Theodicies are not casual matters, and in the weeks after Katrina, they are bound to evolve, even in secular culture, even when they may not resemble the ones that Leibniz had in mind. So they need to be better understood.

The classic theodicies in the West are biblical. The flood of Noah's time, for example, is a reflection of the divine will, cleansing the earth of humanity's evil. A more powerful theodicy later evolved out of the trials of the ancient Israelites, in which destruction and exile were treated not as random accidents of history, but as forms of retribution for violating the Mosaic law and its ethical consequences. Suffering could become proof of divine attention and not its opposite.

Scholars like Norman Cohn have shown how in medieval Europe the worst human trauma could be interpreted as proof of imminent apocalypse and redemption, inspiring millennial expectations and movements. Meanwhile, the theodicy of divine retribution still thrives today and was invoked by some fundamentalist believers after Katrina.

But between medieval Europe and contemporary America something profound changed in the way natural disasters are interpreted and the kinds of theodicies they inspire. And one of the turning points, as many scholars have argued, was the 1755 earthquake in Lisbon. It destroyed perhaps a third of the city's population, with deaths in the tens of thousands. It overturned the confidence of European royalty and seemed to drive a wedge between the earthly and divine realms.

For the growing forces of the Enlightenment, it also seemed to overturn the very idea that a theodicy could account for the disaster. Voltaire, who had once seen nature as benevolent, was whipped into a rationalist fury by the experience. Leibniz, he believed, had been refuted by nature....