How did Plantinga develop the modal ontological argument, and what is the difference between it and Anselm's classical formulation?

This question places us before one of the most exciting developments in contemporary philosophy of religion. Alvin Plantinga, in his book "The Nature of Necessity" (1974), revived an argument that had been considered philosophically "dead" since Kant's critique. But his revival was not mere repetition, but rather a radical reformulation using contemporary modal logic.

Inadequate responses to avoid

From some believers:

"Plantinga proved God's existence mathematically." A dangerous exaggeration. Plantinga himself does not claim that his argument is a "decisive proof," but rather says it shows that belief in God's existence is "rationally acceptable." The difference between "decisive proof" and "rational acceptability" is fundamental in contemporary philosophy of religion.

"The modal ontological argument is mere playing with logical symbols." A reductive misunderstanding. True, the formulation uses symbols from modal logic, but behind the symbols lie deep philosophical ideas about the nature of necessity, possibility, and maximal being.

From some critics:

"All ontological arguments fall into the same error: trying to extract existence from concept." An inaccurate generalization. Plantinga's argument differs structurally from Anselm's argument. It does not claim that "existence is a property," but rather works on the level of "possible worlds" and "logical necessity."

"Modal logic is mere intellectual manipulation unrelated to reality." An extreme position. Modal logic is a recognized philosophical tool, used in metaphysics, analytic philosophy, and philosophy of language. Wholesale rejection of it means rejecting a large part of contemporary philosophy.

Why these responses are inadequate

They share a failure to understand the precise nature of the modal ontological argument, and how it differs fundamentally from classical formulations. Serious evaluation requires understanding modal logic first, then analyzing the argument within its logical framework.

Anselm's classical ontological argument

Saint Anselm (1033-1109) in the "Proslogion" presented two different arguments, but the most famous is the first:

1. God is "that than which nothing greater can be conceived" (id quo maius cogitari non potest).
2. This being exists in the mind at least (even the atheist understands the concept).
3. If it existed only in the mind, one could conceive of a being greater than it: the same being + external existence.
4. But this is a contradiction (a being greater than "that than which nothing greater can be conceived").
5. Therefore, God exists in external reality.

Kant's famous critique

Immanuel Kant in the "Critique of Pure Reason" (1781) presented a devastating critique: "Existence is not a predicate." When we say "God exists," we do not add a new property to the concept of God, but rather affirm that this concept has instantiation in reality. Kant's famous example: 100 conceived thalers = 100 real thalers in terms of conceptual content. The difference lies in external instantiation, not in the concept.

After Kant, the ontological argument was considered philosophically "dead" for nearly two centuries.

The modal logic revolution

In the twentieth century, modal logic was developed by C.I. Lewis and others. This logic deals with concepts of necessity, possibility, and impossibility in a precise formal manner. The basic concepts:

- Possible worlds: different ways reality could be. Our world is one of infinitely many possible worlds.
- Necessity: what is true in all possible worlds (such as: 2+2=4).
- Possibility: what is true in at least one possible world.
- Impossibility: what is true in no possible world.

Plantinga's modal ontological argument

Plantinga reformulated the argument using these tools:

1. Definition: A being possesses "maximal greatness" if it possesses "maximal excellence" in every possible world.
2. Definition: "Maximal excellence" includes omnipotence, omniscience, and perfect goodness.
3. The sole premise: It is possible that a being with maximal greatness exists. (◇∃x[Gx] in modal logic notation)
4. If it is possible that a being with maximal greatness exists, then it exists in at least one possible world.
5. But a being with maximal greatness, by definition, exists in all possible worlds (otherwise it would not be maximally great).
6. Therefore, this being exists in our actual world.

The fundamental difference from Anselm's argument

1. No dependence on "existence as property": Plantinga does not claim that existence is a property added to the concept. The argument works on the level of possible worlds.

2. Only one premise: All the argument needs is acknowledgment that the existence of a greatest being is "logically possible." It does not claim this is obvious or self-evident.

3. Logical transparency: The argument is formulated in the language of formal modal logic, making its structure completely clear.

4. Epistemological modesty: Plantinga does not claim the argument "compels" every rational person, but rather that it shows the reasonableness of belief.

Strengths and weaknesses in Plantinga's argument

Strengths:
- Logical validity: The argument is logically sound. If you accept the premise, the conclusion follows.
- Simplicity: Only one premise, clear and specific.
- Avoiding Kant's critique: Does not depend on considering existence a property.

Weaknesses:
- The main premise is controversial: Is the existence of a greatest being really "logically possible"? The atheist can simply deny this possibility.
- Counter-argument equivalence: One can formulate a similar argument for "necessary non-existence of God" with the same logical structure.
- Modal intuition: The argument depends on our intuitions about what is "possible" and "necessary," and these intuitions may be misleading.

Contemporary debate about the argument

Defenders (Robert Maydole, Alexander Pruss) have developed improved versions and responded to criticism. They emphasize that denying the possibility of a greatest being's existence requires argument, not mere rejection.

Critics (Graham Oppy, Peter van Inwagen, J.L. Mackie) see the argument as depending on a problematic premise. Oppy developed a detailed technical critique in "Ontological Arguments and Belief in God" (1995).

The middle position (Richard Swinburne, Stephen Davis) see the argument as having value within cumulative arguments, but not decisive on its own.

Assessment from the perspective of rational probability (rajḥān ʿaqlī)

The modal ontological argument is an important philosophical achievement that restored academic respect to a type of argument considered dead. But it remains an argument "for specialists" — requiring background in modal logic to understand and evaluate it.

From the angle of rational probability, the argument adds to the cumulative evidence for the reasonableness of belief, especially since it shows that the concept of a "necessary greatest being" is not self-contradictory. But it does not provide "scientific certainty" — which Plantinga himself acknowledges.

Where we stand in this debate today

The modal ontological argument remains a subject of active debate in specialized journals. Recent developments include:
- New formulations using "two-dimensional modal logic" (2D Modal Logic)
- Connecting the argument with theories in metaphysics of divine nature
- Discussion about the relationship between logical possibility and metaphysical possibility

For advanced reading

- Advanced level: The debate between Oppy and Plantinga regarding S5 and the ontological argument
- Alvin Plantinga, The Nature of Necessity (Oxford UP, 1974), Chapter 10
- Graham Oppy, Ontological Arguments and Belief in God (Cambridge UP, 1995)
- The Ontological Argument (collection of essays), ed. Alvin Plantinga (Doubleday, 1965)
- "Family: Ontological Arguments" page on the website