Critics of Leibniz, such as Voltaire, argue that the world contains an amount of suffering too great to justify optimism. While Leibniz argued that suffering is good because it incites human will, critics argue that the degree of suffering is too severe to justify belief that God has created the “best of all possible worlds”. Leibniz also addresses this concern by considering what God desires to occur (his antecedent will) and what God allows to occur (his consequent will). Others, such as the Christian philosopher Alvin Plantinga, criticized Leibniz’s theodicy by arguing that there probably is not such a thing as the best of all possible worlds, since one can always conceive a better world, such as a world with one more morally righteous person.
The Theodicy was deemed illogical by the philosopher Bertrand Russell. Russell argues that moral and physical evil must result from metaphysical evil (imperfection). But imperfection is merely finitude or limitation; if existence is good, as Leibniz maintains, then the mere existence of evil requires that evil also be good. In addition, libertarian Christian theology (not related to political libertarianism) defines sin as not necessary but contingent, the result of free will. Russell maintains that Leibniz failed to logically show that metaphysical necessity (divine will) and human free will are not incompatible or contradictory. He also claims that when Leibniz analyzes the propositions, he is “ambiguous or doubtful…” (O’Briant). Leibniz does not sound sure and is unsure of himself when he writes his premises. He says they do not work together without making Leibniz sound unsure of himself.
Another philosopher who weighs into Leibniz’s philosophy is Kant. Although Leibniz influenced Kant a great deal, Kant found that Leibnizian philosophy “misleading.” (Kant and Early Moderns). He says that the misleading nature of Leibniz’s works is due to the one-sidedness of the theory.