Nothing Is Necessitated Whose Opposite Is

From the Theodicy (1710), Gottfried Wilhelm Leibniz's only book-length work published in his lifetime. The claim is part of his defence of human freedom within a determinist framework: a thing is contingent — not necessitated — when its opposite is logically possible, even if its actual occurrence is determined by sufficient reasons. This allowed Leibniz to argue that God's creation of this world was free (other worlds were possible) while still being certain (God's goodness guaranteed the best possible outcome). The argument drew sustained criticism from Baruch Spinoza's followers and later from Voltaire in Candide.