Article by Jean Le Rond d'Alembert, taken from The Encyclopedia of Diderot and d'Alembert (1751-1773, vol. 11, p. 517; translation J. Iverson).
D'Alembert visited Voltaire at his home outside of Geneva in 1756, so it is quite likely the two men discussed this philosophical concept. At the time, "optimism" was not used in the more general, modern sense of "a positive outlook on life."
Optimism, the name given to philosophers who pretend that this world is the best one God could create, the best of all possible worlds. Father Malebranche and especially Mr. Leibnitz have strongly contributed to gaining acceptance for this school of thought, see Malebranchisme & Leibnitzianism . It is primarily in his Theodicy that the latter of these two philosophers explained and developed his system. It is possible to get some idea of it in his eulogy, published by M. de Fontenelle, Memoirs of the Academy, year 1716 . He maintains, for example, that the crime of Tarquinius who raped Lucretia contributed to the beauty and perfection of the moral world, because this crime produced Roman liberty and, as a consequence, all the virtues of the Roman Republic. But why were the virtues of the Roman Republic necessarily preceded and produced by a crime? This is what we are not told and what it would be quite difficult to explain. And further, how can this optimism be reconciled with the liberty of God, an additional and no less cumbersome question? How is it that so many men slaughter one another in the best of all possible worlds? And if this is the best of all possible worlds, why did God create it? The answer to all these questions consists of two words: o altitudo! &c.* It must be recognized that this metaphysics of optimism is utterly empty.