The modal ontological argument claims that if it is possible that a maximally great being exists, then such a being necessarily exists in reality. The argument employs modal logic to move from the mere possibility of God's existence to its actuality. Its core inferential structure proceeds through three steps: first, it defines a maximally great being as one possessing maximal excellence in every possible world; second, it asserts that such a being's existence is at least possible; third, it concludes that if a maximally great being exists in any possible world, it must exist in all possible worlds, including the actual world. This formulation transforms Anselm's insight about necessary existence into a rigorous modal framework.
The modal version emerged in the 20th century through the work of Charles Hartshorne, who in "The Logic of Perfection" (1962) and "Anselm's Discovery" (1965) reformulated the ontological argument using modal categories. Norman Malcolm's "Anselm's Ontological Arguments" (1960) independently developed similar insights. However, the most influential contemporary version comes from Alvin Plantinga in "The Nature of Necessity" (1974) and "God, Freedom, and Evil" (1974). Plantinga's formulation employs possible world semantics, arguing that maximal greatness entails necessary existence. Robert Maydole's "A Modal Model for Proving the Existence of God" (2003) offers a more recent refinement. These philosophers build on Leibniz's insight that the argument requires demonstrating God's possibility.
The strongest objections target the argument's crucial premise that maximal greatness is possible. Critics like J.L. Mackie in "The Miracle of Theism" (1982) argue that we cannot simply assume the possibility of necessary existence without begging the question. Graham Oppy's "Ontological Arguments and Belief in God" (1995) contends that the argument proves too much—one could construct parallel arguments for necessarily existing evils. Peter van Inwagen suggests the possibility premise requires more justification than intuition provides. Defenders respond that denying God's possibility seems more counterintuitive than affirming it. Plantinga acknowledges the argument may not compel belief but maintains it shows theistic belief is rational. Some defenders argue that the coherence of the concept of maximal greatness provides sufficient grounds for its possibility.
The modal ontological argument differs from other formulations primarily in its explicit use of modal logic and possible worlds semantics. Unlike the Anselmian argument, which moves directly from conceivability to existence, the modal version carefully distinguishes logical possibility from actuality. It differs from the Cartesian version by not relying on the claim that existence is a perfection, instead focusing on necessary existence as essential to maximal greatness. Unlike Gödel's proof, which uses complex mathematical logic, the modal argument remains accessible while maintaining logical rigor.