Plantinga's Modal Ontological Argument

The evidence explained

Plantinga's argument, presented most fully in The Nature of Necessity (1974), defines a 'maximally great being' as one possessing maximal excellence (typically omniscience, omnipotence, and moral perfection) in every possible world, meaning such a being's existence, if possible at all, would be a necessary rather than contingent fact, since necessary existence is itself part of what maximal greatness requires. The argument's crucial and most contested premise is simply that the existence of such a being is possible, that there is at least one possible world in which a maximally great being exists; from this single modal premise, applying the standard logical system S5 for modal logic, the argument validly concludes that a maximally great being exists in the actual world.

Plantinga himself was notably candid about the argument's limitations, stating explicitly that while he considered the argument logically sound (valid, with true premises), he did not regard it as a proof capable of rationally compelling assent from someone antecedently unconvinced, because the key possibility premise, though he argued it was more reasonable to accept than to deny, was not something that could be established independently in a way that would move a genuine skeptic; he described the argument as showing the rationality of theistic belief rather than functioning as a coercive proof.

The central objection, raised in various forms by critics including William Rowe and Graham Oppy, is a parity argument: if one grants that the possibility of a maximally great being's existence entails its actuality, a parallel argument can seemingly be constructed for a 'maximally great island' or other allegedly impossible perfect beings, or, more seriously, for the possibility of a necessarily nonexistent being (a being whose nonexistence is necessary in every world), which, if equally plausible as a starting premise, would generate a contradictory conclusion, suggesting that mere intuitive plausibility of a possibility premise is not sufficient grounds for accepting such powerful modal arguments in either direction.

Defenders including Plantinga respond that maximal greatness, unlike islands (which lack an intrinsic maximum, since more trees, or more coconuts, can always in principle be added), is not obviously subject to the same disanalogy problems, and that the burden remains on the critic to show the possibility premise is actually false or that a truly parallel and equally plausible counter-argument exists, a demand critics argue is not adequately met by Plantinga's disanalogy defense, meaning the debate largely comes down to competing, ultimately intuition-based judgments about whether the initial possibility premise is more reasonable to accept than to reject.