Plantinga's version of the ontological argument claims that if it is possible that a maximally great being exists, then such a being exists in actuality. The argument employs modal logic, specifically S5 modal semantics, to move from the mere possibility of God's existence to its necessity. Plantinga defines a maximally great being as one possessing maximal excellence (omnipotence, omniscience, moral perfection) in every possible world. The inferential structure proceeds: (1) It is possible that a maximally great being exists; (2) If it is possible that a maximally great being exists, then a maximally great being exists in some possible world; (3) If a maximally great being exists in some possible world, then it exists in every possible world; (4) If a maximally great being exists in every possible world, then it exists in the actual world; (5) Therefore, a maximally great being exists in the actual world.
Alvin Plantinga developed this argument in the 1960s and 1970s, presenting its mature form in "The Nature of Necessity" (1974) and "God, Freedom, and Evil" (1974). While Anselm of Canterbury (11th century) originated the ontological approach in his "Proslogion," and modern philosophers like Descartes, Leibniz, and Hartshorne offered variations, Plantinga's contribution was revolutionary in its rigorous use of possible worlds semantics. His formulation emerged during the revival of modal logic in analytic philosophy, particularly through the work of Saul Kripke and David Lewis. Plantinga himself acknowledges the argument's modest aim: not to prove God's existence conclusively, but to show that theistic belief can be rationally acceptable. Contemporary defenders include Joshua Rasmussen, Robert Maydole, and Alexander Pruss, who have refined the argument's modal principles.
The strongest objection targets premise (1): critics argue we have no grounds for asserting that maximal greatness is even possible. Graham Oppy, J.L. Mackie, and Michael Martin contend that possibility claims about necessary beings require justification beyond mere conceivability. The "reverse parody" objection suggests that if Plantinga's reasoning is valid, one could equally prove the existence of a maximally great being's negation. Defenders respond that while we cannot demonstrate the possibility of maximal greatness with certainty, we have intuitive and theoretical reasons to consider it more plausible than its impossibility. Plantinga himself concedes the argument's limitation, stating it establishes only the rational acceptability of theism, not its obligatory acceptance. Some philosophers like Peter van Inwagen argue that the possibility premise enjoys a kind of epistemic presumption, while others like Pruss ground it in broader metaphysical principles about perfection and possibility.
Plantinga's version differs from other ontological formulations primarily in its explicit modal structure and modest epistemic claims. Unlike the Anselmian argument, which moves directly from conceivability to existence, Plantinga carefully distinguishes logical possibility from actuality. The Cartesian version relies on the notion of existence as a perfection, while Plantinga focuses on necessary existence across possible worlds. The Gödelian proof employs mathematical logic to demonstrate God's existence through positive properties, whereas Plantinga uses philosophical modal logic. The Leibnizian version emphasizes the possibility of God through the compatibility of perfections, while Plantinga's argument requires only the bare possibility of maximal greatness.