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Course 14 · Lesson 7.2

Plantinga’s modal ontological argument

9 minute read

Lessons in this course

Course 14 outline

Module 1: Arguments, Existence and Necessity

  1. What natural theology attempts and what it does not
  2. Necessary and contingent existence
  3. How to read and test an argument for God
  4. Philosophy of religion today: a map of the field

Module 2: The Ontological Argument

  1. Anselm and the argument of the Proslogion
  2. Gaunilo and the perfect island objection
  3. Kant: existence is not a real predicate
  4. What the ontological argument would show if it worked

Module 3: Cosmological Arguments

  1. Aquinas: the first three ways
  2. Infinite regress: what is the problem?
  3. Leibniz, the principle of sufficient reason and the argument from contingency
  4. Objections: the brute fact reply and the fallacy of composition

Module 4: The Kalam Cosmological Argument

  1. The kalam tradition: Philoponus, al-Ghazali and Bonaventure
  2. The modern argument: whatever begins to exist has a cause
  3. Did the universe begin? Philosophy and cosmology
  4. Objections and replies, and what follows about the cause

Module 5: Teleological and Fine-Tuning Arguments

  1. From Aquinas’s fifth way to Paley’s watch
  2. Hume and the Dialogues Concerning Natural Religion
  3. Darwin and the change in the design argument
  4. The fine-tuning argument: what is claimed and what is debated
  5. The multiverse and other replies

Module 6: The Moral Argument and Divine Hiddenness

  1. The moral argument: Kant, Lewis and contemporary forms
  2. The Euthyphro question and moral realism without God
  3. Evolution and moral knowledge
  4. The argument from divine hiddenness and Christian replies

Module 7: Contemporary Modal Arguments and Evaluation

  1. Possible worlds and modal logic in plain language
  2. Plantinga’s modal ontological argument
  3. Assessing the modal argument: what it needs and what it shows
  4. Weighing the arguments together, and what none of them prove

All courses

Program / Course 14: Arguments for the Existence of God / Lesson 7.2

What you will learn

  • State Plantinga’s argument in numbered steps.
  • Explain maximal excellence and maximal greatness.
  • Show how the argument uses the idea of possible worlds and S5.
  • Identify the premise that carries the weight and Plantinga’s own claim for the argument.

The big idea

Alvin Plantinga, in The Nature of Necessity (1974) and God, Freedom, and Evil (1974), restated the ontological argument using possible worlds. He defines a maximally great being as one that has maximal excellence in every possible world, and argues that if it is possible that such a being exists, then it exists. The reasoning is valid. The whole question is whether the first premise, that a maximally great being is possible, is true. Plantinga himself says the argument does not prove God to an unbeliever, but shows that belief is rational.

Key terms

Maximal excellence
In Plantinga’s usage, being all-knowing, all-powerful and morally perfect.
Maximal greatness
Having maximal excellence in every possible world. It adds necessity to excellence.
Valid
Describing an argument in which the conclusion must be true if the premises are.
Premise of possibility
The first premise, that it is possible that a maximally great being exists.
Parody argument
An argument of the same form with a contrary first premise, used to show that the form does not favor either conclusion.
Rationally acceptable
Describing a belief it is reasonable to hold, though it may not be provable to everyone.

Background

Plantinga is an American philosopher, born in 1932, a Reformed Protestant who taught at Calvin College and Notre Dame. He reworked an argument by Charles Hartshorne and Norman Malcolm, who had earlier noticed that Anselm’s second argument concerned necessary existence. Plantinga used the formal apparatus of possible worlds to state it clearly. The version below is the one in The Nature of Necessity, in a simplified form.

The argument was widely discussed. Most philosophers found it valid but not convincing, and a large literature grew up on the possibility premise. Plantinga’s own assessment is careful and is important for how the argument should be judged.

Step-by-step explanation

Step 1: The definitions

A being has maximal excellence in a world if it is all-knowing, all-powerful and perfectly good there. A being is maximally great if it has maximal excellence in every possible world. Maximal greatness thus includes necessary existence: such a being cannot fail to exist, because it has to be excellent in every world, which requires it to exist in each.

Step 2: The argument

1. It is possible that a maximally great being exists. 2. If it is possible that a maximally great being exists, then there is a possible world in which a maximally great being exists. 3. If a maximally great being exists in some world, then it has maximal excellence in every world, including that one. 4. So a being exists in that world which has maximal excellence in all worlds, and so in the actual world. 5. Therefore, a maximally great being exists in the actual world.

In effect, the argument uses S5. If it is possible that a necessary being exists, then it is necessary, and therefore actual.

Step 3: Why the argument is valid

Once the first premise is granted, the conclusion follows. Most philosophers, believers and unbelievers alike, accept that the reasoning is valid, given S5. So the question is whether the first premise is true. This is a genuine advance on earlier versions, in which it was unclear what was being assumed.

Step 4: The reverse argument

Critics, including William Rowe and Graham Oppy, note that a parallel argument runs the other way: 1. It is possible that no maximally great being exists. 2. If that is possible, then in some world there is none. 3. A maximally great being would exist in every world if it exists in any. 4. So there is no maximally great being. This argument has the same form and is equally valid. Which first premise is more plausible? The argument does not say, so it cannot show either conclusion unless one of the premises is more reasonable.

Step 5: What Plantinga claims

Plantinga admits that the argument is not a proof that would force an honest person to accept its conclusion. He says that it shows it is rational to accept the conclusion, because the first premise is not less reasonable than its denial, and a person who finds the premise plausible is within epistemic rights to accept it. He calls it successful in showing rational acceptability but not in refuting the opponent. This is a modest claim, and the argument should be judged by it.

  1. Can you state the five steps in your own words?
  2. Why does maximal greatness include necessary existence?
  3. What is the difference between maximal excellence and maximal greatness?
  4. Is the possibility premise more plausible than its denial, in your view?
  5. How does the reverse argument change the situation?
  6. What does it mean to say an argument shows rational acceptability but not proof?

Examples

The bachelor

Compare the statement that there are unmarried husbands. It is not possible, so there is no world with such a person. Plantinga’s first premise says that a maximally great being is possible, as the bachelor is not. The argument therefore depends on whether the concept is coherent.

The reverse argument

Someone argues: it is possible that there is no maximally great being. So there is a world with none. But a maximally great being would be in every world if in any. So there is none. The structure matches Plantinga’s, but it runs the other way. Both cannot be sound.

Plantinga’s own reply

Plantinga holds that the possibility premise is plausible when considered, and that a person has no good reason to deny it. Critics hold that anyone who already denies God will find the denial of the premise equally plausible. Both are within their rights, he says, but only one is true.

Different perspectives

Catholic

Catholic philosophers are divided. Some, in the Thomist tradition, hold that existence cannot be derived from a concept and hence reject all ontological arguments. Others find the modal version interesting. All hold that God’s existence can be known by other arguments, so the modal argument is not required.

Orthodox

Orthodox theology rarely engages the argument. The tradition holds that God exceeds all concepts, and so would not agree that a definition of God as maximally great captures God. The argument is seen as a philosophical exercise.

Protestant

Plantinga is the leading Protestant proponent, and he is joined by some others such as Robert Maydole. Many Protestant philosophers regard the argument as interesting but weak as an apologetic. Barth’s approach would regard it as unnecessary.

Philosophical views

Most philosophers accept that the argument is valid. Some accept the possibility premise. Others, like Oppy, argue that the premise is no more rational than its denial. The argument has a place in textbooks as a major example of modern philosophy of religion.

Skeptical views

Skeptics say that the premise assumes what is to be proved. To grant that a necessary being is possible is, in this context, to grant that it exists. They say that the argument turns a hard question into a harder one.

Why it matters

Plantinga’s argument is the best-known modern version of an ancient idea. Understanding its structure helps in judging many other arguments, because it shows how a valid argument can turn on a single premise, and how a parody can neutralize it.

Common misunderstandings

Misunderstanding: Plantinga claims to have proved God exists.

Correction: He explicitly says that the argument does not prove God to an unbeliever. He says it shows that belief in God is rational.

Misunderstanding: The argument is invalid.

Correction: Most philosophers accept that it is valid, given S5. The question is about its first premise.

Misunderstanding: The reverse argument is a refutation.

Correction: It shows the form does not favor theism by itself. It does not show that the premise of possibility is false.

Misunderstanding: Maximal greatness is just Anselm’s definition in new words.

Correction: It adds explicit reference to every world and to excellence, which makes the logic clearer, but the philosophical issue is similar.

Knowledge check

State Plantinga’s argument.

It is possible that a maximally great being exists. If so, then in some world there is a being with maximal excellence in every world. By the principle that what is possibly necessary is necessary, such a being exists in the actual world. So a maximally great being exists. The argument is valid, and everything turns on the first premise.

What is the reverse argument?

It runs: it is possible that no maximally great being exists; so in some world there is none; but a maximally great being would exist in all worlds if in any; so there is none. It has the same form as Plantinga’s and is equally valid, so the form cannot decide between the two. One must judge which first premise is more reasonable.

What does Plantinga claim the argument achieves?

He says it does not prove God to those who deny the possibility premise. It shows that it is rational to accept the conclusion, because the premise is no less reasonable than its denial. It is therefore successful as a demonstration of rationality, not as a proof that forces agreement.

Why does maximal greatness imply necessary existence?

Because a being with maximal excellence in every world must exist in every world, since a thing that does not exist cannot be excellent. So a maximally great being exists necessarily, if it exists at all. This is how the argument links the possible existence of such a being to its actual existence.

Think it through

  1. What would it take to convince you that a maximally great being is possible?
  2. Does the reverse argument seem as good as the original?
  3. Is rational acceptability a satisfying result?
  4. Does it matter to faith whether this argument works?

Independent assignment

The argument and its mirror

In your own notebook, write 350 to 500 words. Write Plantinga’s argument in numbered steps and write its reverse next to it. Explain in a paragraph why they have the same form, and in another paragraph say which first premise you find more reasonable and why. Do not send this to anyone.

Further study