The ontological argument for the existence of God is one of the oldest and most famous attempts to prove God’s existence through pure reason alone. First formulated by Anselm of Canterbury in the 11th century, it has fascinated philosophers ever since. It sounds simple and powerful: define God as the greatest conceivable being, and from that definition, conclude that God must exist. But like many things that sound too neat, the argument collapses under scrutiny.
Anselm’s Original Argument
The ontological argument is an argument that tries to prove the existence of God through the use of logic. That means it doesn’t rely on external arguments or facts. It is a series of statements that if true, lead to the conclusion that God exists. The argument was first postulated by Anselm, the Archbishop of Canterbury in his book Proslogion in 1078 C.E. Although there are many variations on the argument now, I will focus on the original because most modern variations are quite similar.
Anselm defined God as that than which nothing greater can be conceived. He argued:
- We can conceive of such a being in our minds.
- A being that exists in reality is greater than a being that exists only in the mind.
- Therefore, if God exists only in the mind, we could conceive of a greater being—one that exists both in the mind and in reality.
- This would be a contradiction, since God is by definition the greatest conceivable being.
- Therefore, God must exist in reality.
At first glance, it seems airtight. But philosophers have been poking holes in it for centuries.
Where is the Fallacy?
The philosopher Bertrand Russell once said that “the argument does not, to a modern mind, seem very convincing, but it is easier to feel convinced that it must be fallacious than it is to find out precisely where the fallacy lies.” He was right. When you read it, the argument sounds logical but I think almost anyone will agree that it sounds suspiciously easy or fake. I have even described it as merely word-play and I am still convinced that it is, despite the fact that Anselm was serious when he penned it.
Gaunilo’s “Perfect Island” Objection
Almost immediately after Anselm, his contemporary Gaunilo of Marmoutiers objected with a parody: imagine the perfect island, possessing every perfection an island could have. By Anselm’s logic, such an island must exist, because an island that exists is greater than one that doesn’t. But this is absurd. The structure of Anselm’s reasoning can be used to “prove” the existence of anything, if we just define it as perfect.
The island objection shows that something is wrong with the form of the argument. Simply defining something as existing does not make it real.
Anselm’s Defense
In response to the criticism of Gaunilo and his perfect island, Anselm postulated that God wasn’t just a perfect being, it was also a necessary being, a being that cannot not be. In other words, while Gaunilo’s perfect island need not exist, God has to exist. According to Anselm, this followed from the fact that the universe has a first cause and that this first cause had to be a being.
Kant’s Critique: Existence is Not a Predicate
Centuries later, Immanuel Kant put the most famous nail in the ontological coffin. He argued that existence is not a property that can make something “greater.” To say that a unicorn exists doesn’t add a new property to the concept of unicorn; it simply places the unicorn in reality. Existence is not like being tall, wise, or powerful—it’s not a perfection you can add to a concept. Thus, the idea that “existence makes something greater” is a category mistake.
If existence is not a predicate, then Anselm’s argument fails at its core. You can’t move from a definition in the mind to existence in reality.
Modern Variants: The Modal Ontological Argument
Some modern philosophers, like Alvin Plantinga, reformulated the argument using modal logic. This version says:
- It is possible that a maximally great being exists.
- If it is possible, then such a being exists in some possible world.
- If it exists in some possible world, it exists in every possible world (because it is necessary by definition).
- If it exists in every possible world, it exists in the actual world.
- Therefore, God exists.
This sounds clever but smuggles in the conclusion. Premise 1—“it is possible”—already assumes the coherence of a maximally great being. If the very concept is incoherent (for example, omnipotence and omniscience generating contradictions), then the possibility premise fails. Moreover, defining something into existence via modal logic still faces the same objection as before: you can’t go from definition to reality without independent evidence.
Only Concepts
Another objection that can be made against Anselm’s ontological argument, is the way he describes God as existing in the mind. The problem with that statement is that things do not exist in the mind at all. Rather, minds contain concepts of things, not the things themselves. Therefore, it can’t be said that existing in reality is greater than existing only in the mind for the simple reason that things do not exist in the mind. So one can only say that the concept of a God existing in reality is greater than the concept of a God existing only in the mind, which says nothing about the actual existence or non-existence of God.
Minds contain concepts of things, not the things themselves.
Since the mind only contains concepts, Anselm’s logic doesn’t even get off the ground. There are many concepts which can and do exist in the mind yet we find no examples of them in the real world. For instance, my mind contains the concept of dragons, vampires, werewolves, goblins and many other fantasy creatures. None of these concepts have examples in the real world.
The Concept of God
Another problem with the ontological argument was raised by St. Thomas Aquinas. A formidable theologian and philosopher himself, Aquinas wasn’t convinced by the ontological argument. Aquinas argued that rather than understanding what the concept of “God” means, many people have different concepts of God in their minds. If the ontological argument works at all, it works only for people who have the same understanding of the concept of God.
Problems of Definition
Another problem is the vagueness of “greatest conceivable being.” What does “greatness” mean? Is it moral perfection, infinite power, infinite knowledge, the ability to create worlds, or all of the above? Even if you grant that such a maximal being is conceivable, it’s not clear that the concept is coherent. Can infinite justice and infinite mercy coexist without contradiction? Can an all-powerful being make a stone too heavy for itself to lift? If the very definitions crumble under analysis, the argument collapses.
Necessary Being?
Anselm and modern defenders claim God is not just a being, but a necessary being—one whose non-existence is impossible. But this claim is just asserted, not demonstrated. We can just as easily say, “perhaps the universe itself is necessary” or “perhaps there are necessary truths of mathematics and logic, but no necessary beings.” Unless one can independently establish why God alone must be necessary, the argument is again circular.
The Bigger Problem
Even if the ontological argument worked (it doesn’t), it wouldn’t establish the God of Christianity, Islam, or any religion. At best, it would show that some kind of maximally great being exists. It says nothing about whether this being cares about humans, answers prayers, sends prophets, or judges souls. The jump from “necessary being” to “personal God” is enormous and unjustified.
Conclusion
The ontological argument is elegant, but elegance is not truth. It relies on questionable assumptions: that existence is a property, that a perfect being is coherent, and that we can move from definitions to reality. Gaunilo showed that its form proves too much; Kant showed that its logic misuses existence; modern critics show that modal versions only repackage the same flaw. At best, the ontological argument is a fascinating intellectual puzzle. At worst, it’s a word game dressed up as proof. If God exists, it won’t be because we defined Him into reality.