In the eleventh century, a Benedictine monk named Anselm of Canterbury devised an argument so audacious that philosophers still cannot agree whether it works. He claimed to prove God's existence using reason alone—no appeal to design in nature, no cosmological chain of causes, just pure conceptual analysis.

The ontological argument starts from how we think about God and concludes that God must exist. If this sounds suspicious, you're in good company. Generations of philosophers have felt the same pull of fascination and skepticism. The argument seems to conjure existence from mere definition, like a logical rabbit from a conceptual hat.

Yet the argument refuses to die. Kant thought he'd buried it; modern philosophers have resurrected it using the tools of modal logic. Understanding why this ancient proof keeps returning tells us something profound about the relationship between thought and reality, between what we can conceive and what must be.

Anselm's argument begins with a definition. God is that than which nothing greater can be conceived—the maximally great being. Even atheists, Anselm notes, understand this concept. When they deny God exists, they know what they're denying. So the concept of God exists at minimum in the understanding.

Here comes the crucial move. Anselm argues that existing in reality is greater than existing merely in the understanding. A real hundred dollars can buy things; an imagined hundred dollars cannot. A real friend offers comfort; an imaginary one is lesser. Existence, in this view, is a perfection—something that adds to a thing's greatness.

Now the trap springs shut. Suppose God exists only in the understanding. Then we could conceive of something greater—namely, that same being but existing in reality. But this contradicts our definition: God is that than which nothing greater can be conceived. So the supposition must be false. God cannot exist merely in the mind; God must exist in reality too.

The argument's power lies in its apparent inevitability. Each step seems modest, yet the conclusion is extraordinary. Anselm hasn't shown that a powerful being happens to exist. He's argued that the very concept of maximal greatness logically requires real existence. Denying God's existence, on this view, is like denying that triangles have three sides—a conceptual confusion, not an empirical discovery.

Takeaway

Anselm's argument claims that denying God's existence involves a hidden contradiction—if you truly understand what 'maximally great being' means, you cannot coherently deny that such a being exists.

Immanuel Kant offered what many consider the decisive refutation. His objection is deceptively simple: existence is not a real predicate. When you say 'God is omnipotent,' you add something to the concept of God—the property of unlimited power. But when you say 'God exists,' you add nothing to the concept. You merely posit that the concept has an instance in reality.

Consider the difference between imagining a hundred dollars and having a hundred dollars. The concept is identical in both cases—same denomination, same properties, same potential purchasing power. The real hundred dollars doesn't contain a single cent more in its concept. What differs is whether something in the world corresponds to that concept.

This matters because Anselm's argument treats existence as a property that can make something greater. If existence were like omnipotence or perfect goodness—a quality you can add to a concept—then perhaps existing things would be 'greater' than non-existing things. But existence isn't a quality added to a concept; it's whether the concept is instantiated.

Kant's point undermines the key premise. We cannot say that a God existing in reality is 'greater' than a God existing only in understanding, because we're not comparing two concepts with different properties. We're asking whether a single concept picks out anything real. The greatness-comparison Anselm needs simply doesn't make sense once we see what existence actually is.

Takeaway

Saying something exists doesn't add a property to its concept—it claims the concept has an instance in reality. This means comparing the 'greatness' of existing versus non-existing things rests on a grammatical confusion.

Alvin Plantinga reformulated the argument using possible worlds semantics, sidestepping Kant's objection entirely. A possible world is a complete way reality might have been. The actual world is one possible world among many. Plantinga asks: is there a possible world where a maximally great being exists?

A maximally great being would possess maximal excellence in every possible world—omnipotence, omniscience, and moral perfection necessarily, not contingently. Such a being couldn't be great in one world and mediocre in another. Greatness of this caliber means necessary existence across all possible worlds.

Here's the argument's power. If it's even possible that a maximally great being exists, then there's some possible world where such a being exists. But a maximally great being, by definition, exists in all possible worlds. So if it exists in any possible world, it exists in the actual world. God's mere possibility entails God's actuality.

The debate now shifts to the key premise: is a maximally great being genuinely possible? The argument is logically valid under S5 modal logic. But critics question whether we can know the concept is coherent. Perhaps maximal greatness is subtly contradictory, like a married bachelor. The ontological argument hasn't disappeared—it's transformed into a question about whether perfect being theology describes something genuinely conceivable or a sophisticated impossibility.

Takeaway

Modern versions of the argument avoid the 'existence isn't a predicate' objection but face a new challenge: proving that maximal greatness is genuinely possible, not just apparently conceivable.

The ontological argument remains philosophy's most controversial proof—too elegant to ignore, too strange to fully accept. It forces us to confront fundamental questions about what logic can tell us about reality.

Whether you find Anselm brilliant or Kant decisive, the argument sharpens our thinking about existence, predication, and possibility. It reveals that our concept of God isn't like other concepts—if coherent at all, it carries extraordinary metaphysical implications.

Perhaps that's the argument's lasting value. Even if it doesn't prove God exists, it demonstrates that the question of God's existence isn't empirical in the way existence questions usually are. What we can coherently conceive may constrain what can possibly be.