Theodicy

The only full-length philosophical book Leibniz published in his lifetime, written in French and coining the word 'theodicy', the justification of God. It is above all a reply to Pierre Bayle, who had argued that reason cannot reconcile the evil in the world with a good and all-powerful God and that faith must stand on its own. A preliminary discourse argues that faith and reason cannot truly conflict. The three main parts then answer Bayle. God, choosing among infinitely many possible worlds, must have chosen the best, and the best is not a world without evil but the one with the greatest overall perfection. Leibniz distinguishes metaphysical evil, the limitation of every creature, from physical evil, or suffering, and moral evil, or sin. He defends human freedom against necessity by separating what is certain from what is necessary. The book ends with a fable of Sextus Tarquinius and a palace of possible worlds. Voltaire mocked its optimism in Candide.

It is the classic statement of the best-possible-world answer to the problem of evil, and still the reference point for anyone who argues about why a good God would permit suffering.