Leibniz co-invented calculus independently of Newton, pioneered symbolic logic, and built one of the most ambitious metaphysical systems of the early modern period. His Theodicy (1710) — the only book he published in his lifetime — coined the word “theodicy” and defined the problem for modern philosophy: how can God be omnipotent, omniscient, and perfectly good if evil exists? His answer, that we live in “the best of all possible worlds,” was a precise metaphysical argument, not naive optimism — though it became famous for being read as one. Voltaire’s Candide is essentially a sustained attack on this claim, triggered by the Lisbon earthquake of 1755. Whether Leibniz was right or wrong, he set the terms that every subsequent theodicy has had to engage.