What is the ontological argument for God's existence?

Of all the classical arguments for God, the ontological argument is the strangest to encounter for the first time. It does not appeal to the universe, to design, or to morality. Instead, it starts entirely with a definition and tries to show that the definition itself requires God's existence. Many people find it either instantly persuasive or completely baffling, and philosophers have debated it for nearly a thousand years.

It was first developed by Anselm, Archbishop of Canterbury, in the eleventh century, and later refined by philosophers like Descartes and, in the twentieth century, Alvin Plantinga using the tools of modal logic.

Anselm defined God as 'that than which nothing greater can be conceived'—in other words, the greatest possible being, the maximum of every good quality. He then argued that existing in reality is greater than existing merely as an idea in the mind. If God existed only as a concept, then we could conceive of an even greater being: one that actually exists.

But since God is by definition the greatest conceivable being, he cannot merely be an idea; he must exist in reality, otherwise the definition contradicts itself. This is a logical, not empirical, argument—it tries to show that denying God's existence is, in a sense, self-defeating.

Alvin Plantinga's version reframes the argument using the concept of possible worlds. A 'maximally great being' would need to exist not just in our world but in every possible world—meaning existence would be part of its very nature, not an added feature. If it is even possible that such a being exists, then, by the logic of the argument, such a being exists necessarily, in all possible worlds, including our own.

This is the argument's most controversial step, since much depends on granting that a maximally great being is indeed possible. Even critics who reject the argument's force often admit it is logically valid, which has kept philosophers engaged with it for centuries.

The philosopher Immanuel Kant famously objected that existence is not a 'predicate'—not a property that makes a thing greater, the way redness or strength would. On this view, saying something exists doesn't add a quality to its definition, so the argument's key move fails. Others argue you could use the same logic to 'prove' a perfect island or a perfect unicorn exists, which seems absurd.

Defenders respond that necessary existence is different from ordinary existence and cannot be applied to contingent things like islands, which by nature could always fail to exist. The debate remains genuinely unresolved among philosophers, which is why most apologists treat this argument as supplementary rather than foundational.

This 'perfect island' objection actually predates Kant by seven centuries; it was first raised by a monk named Gaunilo, a contemporary of Anselm himself, who wrote a direct reply defending the idea of a 'lost island' more excellent than any other island. Anselm's own response was that the argument works uniquely for a maximally great being, not contingent objects like islands, because islands have no intrinsic maximum of greatness—you could always imagine one slightly better, with one more palm tree or a slightly nicer beach. A maximally great being, by contrast, is not a category with an indefinite upper limit; 'greatest conceivable' functions as a genuine ceiling only when applied to a being who possesses qualities like power, knowledge, and goodness to their logical maximum.

Centuries after Anselm, René Descartes offered his own version of the ontological argument, reasoning that the very idea of a perfect being includes existence as one of its perfections, much as the idea of a triangle includes having three angles that sum to 180 degrees. To conceive of a perfect being lacking existence, Descartes argued, would be as contradictory as conceiving of a three-sided triangle with only two angles. Whether or not this analogy fully succeeds is still debated among philosophers, but it illustrates a broader theological approach sometimes called 'perfect being theology'—reasoning about God's nature by asking what qualities the greatest possible being must have.

This approach has shaped much of historic Christian theology, even among thinkers who never mention Anselm by name. When Scripture says of God, 'his greatness no one can fathom' (Psalm 145:3), it is affirming exactly the kind of maximal, unsurpassable greatness the ontological argument tries to formalize with logic. The philosophical exercise and the psalmist's worship, in other words, are reaching for the same reality from different directions.

The Bible does not argue for God's existence the way a philosopher would; it simply announces him: 'In the beginning, God' (Genesis 1:1). When God reveals his name to Moses, he says, 'I am who I am' (Exodus 3:14)—a name that points to self-existence, to a being whose very nature is to exist, needing no cause and depending on nothing else.

Interestingly, this lines up with the philosophical intuition behind the ontological argument: God is not one being among many contingent things but the necessary ground of all existence, the 'I AM' who cannot not be.

You will probably never win someone to Christ with the ontological argument alone; it is too abstract for most everyday conversations. But it is worth understanding because it shows that belief in God has occupied history's sharpest philosophical minds, not merely the naive or unquestioning.

The psalmist offers a simpler but no less pointed observation: 'The fool says in his heart, "There is no God"' (Psalm 14:1)—not because atheists lack intelligence, but because denying the Being who grounds all reality is, in the end, a denial of something woven into the very fabric of thought itself.