An ontological argument is an a priori argument based entirely on reason. The Latin phrase a priori literally means “from the former” but is used to mean without prior experience. In other words, an a priori argument is not based on any previous experience, observation, or evidence but is self-evidentially true. Thus, an ontological proof for the existence of God is any argument that seeks to prove that God exists from nothing but the concept of God itself.

The medieval theologian and Archbishop of Canterbury, St. Anselm, was the first to make such an argument for God’s existence in the beginning of his book, Prosologian. His argument can be summarized as follows:

1. God is defined as a being that “which none greater can be conceived.”
2. The concept of God exists in human understanding.
3. Assume that God does not exist.
4. The concept of God existing in reality exists in human understanding.
5. An entity which exists in reality is greater than an entity that exists only in our understanding.
6. Because it was assumed that God does not exist, the entity in (5) would be greater than God, the being that which none greater can be conceived, which is a contradiction.
7. Conclusion: The assumption in (3) must be incorrect, and therefore, God must exist.

This type of proof is a proof by contradiction, or an argument by reductio ad absurdum, which is a valid form of argumentation. If you assume the opposite of what you want to conclude (God doesn’t exist) and it can be shown that this assumption leads to a contradiction (step 6), then the original assumption must be false. Yet intuitively there still seems like there is something wrong with this argument, even if it is not initially clear what that is. After all, it appears that Anselm is simply defining God into existence! Or, does a perfect being exists by virtue of the fact that it’s a perfect being?

One of Anselm’s contemporaries, a monk named Gaunilo, was one of the first writers to record an objection to Anselm’s argument. Gaunilo argued that Anselm’s proof could be used to “prove” the existence of all sorts of “perfect” things like, for instance, a perfect island. A perfect island that exists in reality is greater than a perfect island that exists only in the mind, therefore, that perfect island must actually exist in reality! Defenders of Anselm’s argument were able to overcome Gaunilo’s objection by arguing that we don’t really have any understanding of what a “perfect” island would entail while we do have a clear understanding of what a perfect being would entail (infinite in all capacities). However perfect of an island you can imagine, somebody else could always come up with one that’s just a bit more perfect. Because there is no real idea of perfection when attached to the notion of an island, the argument can no longer be applied to the island.

The philosopher Immanuel Kant raised one of the more important objections to the Ontological argument in hisCritique of Pure Reason . Kant argued that existence is not a property of things that can be added to other properties describing the being to make it “greater.” To say that some object X has the property Y is to say that If X existed, then X would be like Y. If existence was a property, then this would mean nothing more than If X existed, then X would exist, which must be true. Furthermore, if existence and non-existence can be properties of objects, then if an object has the property of non-existence this would mean that it has no properties (because it doesn’t exist), which is contradictory. Thus, existence cannot be a property of God that makes It a greater being – either it exists and has properties, or it doesn’t exist and has no properties.

Another criticism of Anselm’s argument focuses on premise number 2, “The concept of God – a being that which none greater can be conceived – exists in human understanding.” It can be argued that in reality we don’t have any real understanding of such a being, that is, we don’t actually know what  a being which none greater can be conceived would actually be.

Finally, one could also point out that Anselm seems to be confusing the idea with the reality the idea refers to. Even if existence must be a part of the idea of a perfect being, this does not prove that such a being actually exists in reality behind the idea. We can only ever conceive of anything as existing, regardless of whether or not it truly exists in reality, for to “conceive” of something as non-existing would be to conceive of nothing at all.

Descartes’ Ontological Proof

The mathematician and philosopher René Descartes formulated his own version of an ontological argument in his Meditations on First Philosophy. His argument differs from Anselm’s in that it does not argue to the existence of God from the concept of God itself, rather, it argues to the existence of God from the existence of a clear and distinct idea of a supremely perfect being. Descartes argument is simply this:

1. Every idea must be caused, and the cause must be as real as the idea. If I have any idea of which I cannot be the cause, then something besides me must exist.
2. All ideas of the material reality could have their origin within me. But the idea of God, an infinite and perfect being, could not have originated from within me, since I am finite and imperfect.
3. I have an idea of God, and it can only have been caused by God.
4. Conclusion: God must exist.

It goes without saying that this argument is just as hard to swallow as Anselm’s. One certainly does not have to grant to Descartes that the idea of a perfect and infinite being could not have originated from within man’s own imagination.

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