The Ontological Argument for the Existence of God

Author: Andrew Chapman
Category: Philosophy of Religion
Word Count: 1000

1. God’s Greatness

The Abrahamic conception of God is that he’s awesome—all-powerful, all-knowing, all-good, creator of the universe, self-existent, and a host of other properties that make him not just very, very great, but the greatest that there is or could possibly be.

“This is all fine and good,” say non-theists, “but this is a description of a being whose existence we don’t affirm.” However, a famous and powerful argument for God’s existence known as the Ontological Argument purports to be able to show that God’s being the greatest possible being entails God’s existence: the mere definition of God proves his existence.

This essay introduces this argument. 


2. Anselm’s Ontological Argument

There are different versions of the Ontological Argument, but we will focus on one of the earliest, set forth by St. Anselm (1093-1109).1

As we’ve already noted, God is the being than which no greater can be conceived. This is Anselm’s somewhat unwieldy description of God, which we can abbreviate as BNGC. By definition, BNGC is the greatest conceivable being. If you think you’re conceiving of God and you can possibly conceive of a greater being, then you weren’t initially conceiving of God. Simple enough.

Now, certainly you can conceive of God. To conceive of something is just to think about it clearly and distinctly; you’ve been doing that since the beginning of this essay. So we know, at least, that God can exist in conception, i.e., can be conceived. Even atheists admit this. What the atheist is denying, and what the agnostic is refusing to affirm or deny, is that God exists in reality. So we have an intuitive distinction between a thing that exists merely in conception and a thing that exists in reality as well as in conception.

Now here’s the argument: Assume that the atheist is right, that God doesn’t exist in reality, but merely in conception. But then there would be another possible being, a God who exists not merely in conception but also in reality as well, who is greater than BNGC.2 That is, there would be a possible being who is greater than the being than which no greater can be conceived. But no being can be greater than the being than which no greater can be conceived—that’s a flat-out contradiction! So our original assumption, that God doesn’t exist in reality, but merely in conception, must be false, since any assumption that entails a contradiction must be false. Therefore, God must exist both in conception and in reality. Therefore: God exists.3

The Ontological Argument is remarkable in that it reasons from premises containing only definitions and logical laws to perhaps the grandest philosophical conclusion there is. We can know that God exists merely by reflecting on the concept of God.

Many people, however, have been uncomfortable with the purported fact that we can prove the Almighty’s existence so apparently simply. Numerous critics, theist and non- alike, have criticized different aspects of the Ontological Argument. Let’s look at just two of the most influential criticisms: those provided by Gaunilo of Marmoutiers (994-1083) and Immanuel Kant (1724-1804).

3. Gaunilo’s Criticism

Gaunilo was a monk and a contemporary of Anselm’s. In his “Reply on Behalf of the Fool,”4 Gaunilo has us imagine another really awesome thing: the island than which no greater can be conceived—let’s call it INGC. This island has all the amazing-making properties you can think of: pristine white-sand beaches for lounging, warm water for swimming, and not a tourist in sight. But certainly such an island’s existing only in conception would entail a contradiction, since then there would be a possible thing greater than the INGC, namely, the existing INGC. Therefore, the INGC exists. And, of course, since we have picked island arbitrarily, we can run the same argument for any object: a building, a mousetrap, a horse, whatever you please.5

What Gaunilo has shown, then, is that, using Anselm’s form of reasoning, we can prove the existence of all sorts of bizarre entities, entities that clearly don’t exist. Accordingly, concludes Gaunilo, there must be something fatally wrong with Anselm’s reasoning.6

4. Kant’s Criticism

Which do you prefer, coffee or existing coffee? Notice that this is different from the question of whether you prefer coffee or no coffee at all. No coffee isn’t coffee while both coffee and existing coffee are coffee just the same! If it seems like we’re verging on Lewis Carroll-style nonsense here, you’re right, and this is exactly Kant’s criticism of the Ontological Argument.

According to Kant in his Critique of Pure Reason, what’s gone wrong with Anselm’s argument lies in the distinction between a thing that exists merely in conception and a thing that exists in reality as well as in conception. According to Anselm, there are two different sorts of things: those that exist merely in conception and those that exist in reality as well as in conception. But an existing thing and its non-existing counterpart aren’t two different sorts of thing—one merely exists and the other doesn’t. While it is certainly true that some things exist and others do not, existing does not make a thing a different kind of thing from its non-existing colleague.

The upshot of this, says Kant, is that existence is a very special type of property, one not suited for the type of argument Anselm is running. Since there is no difference between the group of objects falling into the class God and those falling into the class existing God, an existing God can be no better and no worse than a mere God. There’s simply no relevant difference in kind between a God who exists and a God who doesn’t.

5. Conclusion

Of course, Gaunilo and Kant have not had the last word in this debate. Powerful arguments have been mounted in response to Gaunilo’s and Kant’s criticisms of the Ontological Argument. Additionally, increasingly complex versions of the Ontological Argument have been developed and debated. One thing that’s certain is that the Ontological Argument, whether sound or unsound, is a fascinating and powerful attempt at a proof for the existence of God.


Two other famous formulations of the argument are Descartes’s formulation from the conception of existence as a perfection (see Descartes’ Meditations 4-6 by Marc Bobro) and Alvin Plantinga’s so-called Modal Ontological Argument (see Modal Ontological Arguments for the Existence of God by Thomas Metcalf). 

Which is greater, a God who exists merely in conception or a God who exists in reality as well as in conception? Think of all the things a God who exists in reality as well as in conception can do that a God who exists merely in conception cannot do: He can create worlds. He can listen to prayers. He can be the ultimate source and ideal form of goodness. He can reward virtuousness and punish vice… Those all seem like great things, and a God who exists merely in conception can do none of them.

You may remember this type of argument or proof from your geometry courses where it was called an indirect proof. Philosophers and logicians call this a reductio ad absurdum, or a reduction to absurdity. The strategy, as you have seen, is to assume the opposite of what you are trying to prove, show how that assumption entails either a contradiction or some other form of absurdity, and then to reject the original assumption.

“The fool has said in his heart, ‘There is no God.’” (Psalm 14:1)

It’s important to note that we’re not merely talking here about the greatest actually existing island, mousetrap, horse…, but the greatest possible island, mousetrap, horse… It is plausible that for any type of existing object, one of the ones that exists is the best one (in terms of whatever makes that sort of thing a good one of what it is). But it is another thing altogether to talk about the greatest possible or greatest conceivable such object.

Notice that Gaunilo’s argument is also a reductio ad absurdum: Assume that Anselm’s reasoning is valid and an absurdity results. Therefore, Anselm’s reasoning must be flawed.


Anselm, St., Proslogion, in St. Anselm’s Proslogion, M. Charlesworth (ed.), Oxford: OUP, 1965.

Descartes, R., Discourse on Method and The Meditations, translated with an introduction by F. Sutcliffe, Harmondsworth: Penguin, 1968.

Gaunilo, “On Behalf of the Fool”, in St. Anselm’s Proslogion, M. Charlesworth (ed.), Oxford: OUP, 1965.

Kant, Immanuel. Critique of Pure Reason. Trans. Paul Guyer and Allen W. Wood. Cambridge: Cambridge UP, 1998.

Oppy, Graham. “Ontological Arguments.” Stanford Encyclopedia of Philosophy. Stanford University, 08 Feb. 1996. Web. 27 June 2014. 

Related Essays

The Concept of God: Divine Attributes by Bailie Peterson

Descartes’ Meditations 4-6 by Marc Bobro

Modal Ontological Arguments for the Existence of God by Thomas Metcalf 

Possibility and Necessity: An Introduction to Modality by Andre Leo Rusavuk

Modal Epistemology: Knowledge of Possibility & Necessity by Bob Fischer

About the Author

Andrew Chapman is a lecturer in philosophy at the University of Colorado, Boulder. He holds a PhD in philosophy from the University of Colorado, Boulder, an MA in philosophy from Northern Illinois University and a BA in philosophy and a BM in bassoon and sound recording technology from Ithaca College. He specializes in epistemology, metaethics, and the history of philosophy (especially Kant and the 20th Century Anglophone and Phenomenological traditions). When not philosophizing, Andrew is skiing, hiking, listening to great music, or playing the bassoon. 

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