Natural Theology • Existence of God • Advanced
Ontological Argument
Primary question: Can the very concept of God as the greatest conceivable being, or as a maximally great being, entail that God actually exists?
Argument map
Click any node to inspect it. Expand branches to go deeper.
Common mistakes
Don't say
Assuming the ontological argument tries to derive existence from a mere stipulated definition of any concept.
Instead
Clarify that it targets a very specific concept — maximal greatness or supreme perfection — argued to uniquely include existence or necessary existence as part of its content.
Don't say
Treating Gaunilo's Perfect Island parody as a decisive refutation without addressing the disanalogy response.
Instead
Explain that defenders distinguish intrinsically-maximal qualitative perfections from open-ended quantitative properties like island features.
Don't say
Presenting Plantinga's modal version as if it 'proves' God exists to any rational person.
Instead
Note that Plantinga himself frames the argument as showing theistic belief can be rational, since the entire weight rests on the contested possibility premise.
Don't say
Ignoring Kant's objection that existence is not a predicate as if it were fully answered.
Instead
Acknowledge Kant's objection remains a serious challenge to Anselm's and Descartes's classical versions, even if modal versions attempt to sidestep it.
What would change my mind? — Christian position
A rigorous proof that 'necessary existence' is an incoherent modal property, or a fully general parody with no relevant disanalogy, would substantially undermine confidence in the argument.
What would change my mind? — Skeptical position
A demonstration that the concept of maximal greatness is coherent and that its possibility is more plausible than its impossibility, absent question-begging, would make the modal argument far more compelling.
Why does this matter?
The ontological argument matters because, unlike cosmological or teleological arguments, it purports to reach God's existence through pure reason alone, independent of any empirical observation about the world. If successful, it would show theism to be a truth of reason akin to mathematics; if it fails, its failure illuminates deep questions about the relationship between concepts, possibility, and existence that recur throughout metaphysics.
For further study
Primary and secondary sources on both sides of this question. Reading the strongest opposing case is part of the work, not a concession.
Proslogion — Anselm of Canterbury
The original 11th-century source of the argument.
Meditations on First Philosophy — René Descartes
Presents the perfection-based version in Meditation V.
The Nature of Necessity — Alvin Plantinga
Canonical presentation of the modal ontological argument.
Critique of Pure Reason — Immanuel Kant
Contains the classical 'existence is not a predicate' objection.
Ontological Arguments and Belief in God — Graham Oppy
Comprehensive critical survey of ontological arguments and their history.