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Modal Ontological Argument

The Modal Ontological Argument is a modern reformulation of Anselm’s reasoning, most famously developed by Alvin Plantinga, using modal logic (the logic of possibility and necessity).

Main Idea

  1. Definition of a Maximally Great Being (MGB)
    • A being that has maximal excellence (all-powerful, all-knowing, morally perfect) in every possible world in which it exists.
    • If such a being exists, it would be necessary — it cannot fail to exist.
  2. Possibility Premise
    • It is possible that a maximally great being exists.
    • In modal logic, if something is possible, it exists in at least one possible world.
  3. Necessity Premise
    • If a maximally great being exists in any possible world, by definition, it exists in every possible world (since it must be necessary, not contingent).
  4. Conclusion
    • Therefore, if it is even possible that such a being exists, it must exist in all possible worlds — including the actual world.
    • Thus, God (as a maximally great, necessary being) exists.

Here’s the Modal Ontological Argument structured without the diagram, keeping the full syllogism and objections for easy study.

1. The Modal Ontological Argument (Formal Syllogism)

  1. Definition
    • A Maximally Great Being (MGB) is a being with maximal excellence (omnipotence, omniscience, moral perfection) in every possible world in which it exists.
  2. Premise 1 – Possibility
    • It is possible that a Maximally Great Being exists.
  3. Premise 2 – Existence in Some World
    • If it is possible for such a being to exist, then it exists in some possible world.
  4. Premise 3 – Necessity
    • If a Maximally Great Being exists in some possible world, then, by definition, it must exist in every possible world (because a maximally great being cannot be contingent).
  5. Conclusion
    • Therefore, a Maximally Great Being exists in every possible world, including the actual world.
    • Therefore, God exists necessarily.

2. Key Objections and Responses

1. Possibility Premise Challenge

  • Objection: The argument hinges on whether it’s actually possible for such a being to exist. If the concept is incoherent (like a square circle), the argument collapses.
  • Response: Defenders argue omnipotence, omniscience, and moral perfection are not logically contradictory, so the possibility premise stands unless proven otherwise.

2. Maximally Great Pizza (or Island) Parody

  • Objection: Couldn’t we “prove” a perfect pizza or island exists in all possible worlds?
  • Response:
    • Pizzas and islands are material and inherently contingent.
    • They admit limitless “improvements,” so they lack intrinsic maximal greatness.
    • Only a being with necessary existence and essential attributes (power, knowledge, morality) fits the structure.

3. “Existence by Definition” / Begging the Question

  • Objection: The argument seems to smuggle existence into the definition, assuming what it seeks to prove.
  • Response:
    • It doesn’t assume actual existence, only possible existence.
    • The modal reasoning (if possible, then necessary) is valid; the real debate is over the possibility premise.

4. Competing “Necessities”

  • Objection: Could someone define a “necessarily non-existent” being to counter the argument?
  • Response:
    • “Necessary nonexistence” is incoherent—nonexistence cannot be a “great-making” property.
    • Necessary existence, unlike contingent existence, is a coherent modal property.