What is the significance of the Lisbon earthquake in Candide?

The 1755 Lisbon earthquake, tsunami, and resulting fires of All Saints' Day, had a strong influence on theologians of the day and on Voltaire, who was himself disillusioned by them. The earthquake had an especially large effect on the contemporary doctrine of optimism, a philosophical system which implies that such events should not occur. Optimism is founded on the theodicy of Gottfried Wilhelm Leibniz that says all is for the best because God is a benevolent deity. This concept is often put into the form, "all is for the best in the best of all possible worlds" (Fr. "Tout est pour le mieux dans le meilleur des mondes"). Philosophers had trouble fitting the horrors of this earthquake into the optimist world view. This is why Candide rejected his early indoctrination with Leibnizian optimism (or simply Optimism) by his mentor, Pangloss.