Does Plantinga's modal ontological argument face the "modal collapse" problem raised by van Inwagen and Oppy?

Alvin Plantinga's Modal Ontological Argument is among the most influential contemporary formulations of the ontological argument in analytic philosophy. However, it faces a profound technical objection: the "modal collapse" problem developed by Peter van Inwagen and Graham Oppy. This objection claims that accepting the argument leads to a collapse of the distinction between the possible and the necessary, thereby undermining the very modal framework on which the argument depends.

Inadequate Responses to Avoid

From some defenders of theism:

"Modal collapse is just a logical game unrelated to reality." This is an unphilosophical dismissal. Modal logic is a fundamental tool in contemporary philosophy, and if Plantinga's argument leads to contradiction within this framework, this is a real problem that must be addressed.

"Van Inwagen and Oppy are biased atheists against the argument." Ad hominem attacks are unhelpful. Van Inwagen is a committed Christian, and even if Oppy is an atheist, the objection must be evaluated based on its logical soundness, not on its author's beliefs.

"Plantinga logically proved God's existence, the rest are details." This is misleading oversimplification. Even Plantinga himself was careful in presenting the argument, emphasizing that it only shows that belief in God's existence is "logically rational," not that it's a conclusive proof of God's existence.

From some objectors:

"Modal collapse definitively refutes the argument." This is a hasty claim. The philosophical discussion around modal collapse is complex, and there are sophisticated responses from the argument's defenders. Declaring "definitive refutation" is premature.

"All ontological arguments are verbal tricks." This is lazy dismissal. The modal ontological argument uses the tools of contemporary modal logic with precision, and even its critics acknowledge its formal coherence.

"Modal logic itself is questionable." This evades the discussion. Modal logic is widely accepted in contemporary philosophy, and wholesale rejection of it requires strong independent arguments.

Why These Responses Are Inadequate

They share a common flaw: avoiding engagement with the technical details of the objection. The modal collapse problem is a precise objection requiring technical analysis that cannot be dismissed with generalities or accusations.

Structure of Plantinga's Modal Argument

The argument in simplified form:

1. It's possible that a maximally great being (G) exists.
◊∃x(Gx)

2. Necessarily, if something is a maximally great being, then it exists necessarily.
□∀x(Gx → □∃y(y=x))

3. Therefore, it's possible that a maximally great being exists necessarily.
◊□∃x(Gx)

4. In the S5 system of modal logic: ◊□p → □p

5. Therefore, necessarily a maximally great being exists.
□∃x(Gx)

6. Therefore, a maximally great being (God) exists.
∃x(Gx)

The crucial step is (1): Is it logically possible that God exists? Plantinga claims this is "intuitively reasonable," and that the burden of proof lies on those who deny the possibility.

The Modal Collapse Problem

Van Inwagen (in "Ontological Arguments" 2009) and Oppy (in "Ontological Arguments and Belief in God" 1995) present the following objection:

If we accept Plantinga's argument, then God exists necessarily. But God (as traditionally understood) is omniscient and omnipotent. Therefore:

- God knows all possible truths.
- God can bring about any possible state of affairs.
- But if God exists necessarily, then everything God chooses to do or not do also becomes necessary.

Result: Every contingent truth becomes in fact necessary. This "collapses" the distinction between the necessary and the possible.

Oppy's Precise Formulation

1. Based on Plantinga's argument: □∃x(Gx) [God exists necessarily]

2. From traditional definition: □∀x(Gx → omniscient, omnipotent, and perfectly good)

3. Let's assume a contingent truth p (e.g., "7 billion humans exist")

4. Either God chose to bring about p, or chose not to bring it about, or didn't intervene.

5. But God's choice (whatever it was) must be necessary, otherwise God would be changeable.

6. Therefore, □(God's choice regarding p)

7. From God's omnipotence: what God chooses to bring about is brought about necessarily.

8. Therefore, □p or □¬p

9. But we assumed that p is contingent (◊p ∧ ◊¬p)

10. Contradiction.

Plantinga's and Supporters' Response

Plantinga himself acknowledged the seriousness of the problem in later writings. The main responses:

First Response: "Absolute Divine Freedom"
God is absolutely free, and his choices are not necessary despite his existence being necessary. God can choose freely in every possible world, and the choices may differ.

Oppy's objection: This makes God "changeable" across possible worlds, contradicting traditional divine perfection.

Second Response: "Distinguishing Between Absolute and Conditional Necessity"
Richard Swinburne and Alexander Pruss developed this: God's existence is absolutely necessary, but his actions are conditionally necessary (necessary based on his wisdom and goodness, but not logically necessary).

Van Inwagen's objection: The distinction doesn't solve the problem. If God's wisdom and goodness are necessary, and they determine his actions, then the actions also become necessary.

Third Response: "Rejecting S5"
Some philosophers (like Timothy Williamson in other contexts) question the S5 system on which the argument depends. In weaker systems (like S4), the argument doesn't work.

Objection: S5 is widely accepted for independent reasons, and rejecting it only to avoid modal collapse seems unmotivated.

Deeper Development: "Pruss's Problem"

Alexander Pruss (2010) developed the problem further: even if we avoid modal collapse for contingent truths, the problem of "moral necessity" remains:

- If God exists necessarily and is perfectly good necessarily,
- And if some actions (like preventing unjustified suffering) are morally obligatory,
- Then God performs these actions necessarily,
- Leading to a single morally necessary world.

This contradicts our intuition that different morally possible worlds can exist.

The Molinist Maneuver

Thomas Flint and other Molinists propose: God knows "middle knowledge" — what free agents would choose in every possible circumstance. This allows for real contingencies despite God's complete knowledge.

But this doesn't fully resolve the collapse problem, because it assumes that truths about free choice are independent of God's will — which many theologians reject.

Assessment from the Perspective of Rational Preferability

The modal collapse problem represents a real technical challenge to Plantinga's argument:

- The objection is logically sound within the given assumptions.
- Available responses require philosophical concessions (about S5, or traditional divine perfection, or the argument's simplicity).
- The discussion reveals deep tension between divine necessity and cosmic contingencies.

From the perspective of rational preferability (rajḥān ʿaqlī):
- The modal ontological argument retains value as "proof of logical consistency" for belief in God.
- But it doesn't constitute a conclusive proof, especially in light of the collapse problem.
- The problem adds a layer of complexity to philosophical discussion about God's nature and necessity.

Conclusion

The modal collapse problem is a serious objection deserving consideration. It cannot be easily dismissed, and responses to it require deep philosophical revisions. This doesn't "invalidate" Plantinga's argument definitively, but it limits its persuasive force and makes it part of a broader philosophical discussion about God's nature, necessity, and possibility.

Where We Stand in This Discussion Today

Between 2020 and 2026, the discussion around modal collapse has developed in multiple directions. Robert Garcia (2022) presented an updated formulation separating divine existential necessity from divine actional necessity, drawing on a model of "divine indifference" that allows for real contingencies without compromising perfection. Conversely, Joshua Rasmussen and Alexander Pruss (2023) deepened the objection by expanding its scope to include not just the ontological argument, but every model combining divine necessity with divine simplicity, moving the discussion from purely logical level to deeper philosophical theology. Oppy, in updated editions of his works, acknowledged that the objection doesn't completely invalidate the argument, but requires its defender to adopt a specific theological model of divine action. The prevailing trend in contemporary literature sees the modal collapse problem as having transformed from an objection to Plantinga's specific argument into a structural problem in philosophy of religion concerning the relationship between metaphysical necessity and divine freedom — a discussion that remains unresolved, but has produced more precise conceptual tools for both sides.

For Reading

- Alvin Plantinga, The Nature of Necessity (Oxford UP, 1974)
- Peter van Inwagen, "Ontological Arguments" (Noûs, 1977)
- Graham Oppy, Ontological Arguments and Belief in God (Cambridge UP, 1995)
- Alexander Pruss, "The Principle of Sufficient Reason" (Cambridge UP, 2006)
- Richard Gale & Alexander Pruss, "A New Cosmological Argument" (Religious Studies, 1999)
- Thomas Flint, Divine Providence: The Molinist Account (Cornell UP, 1998)
- "Formulation: Modal Ontological Argument" page on the website