St. Anselm’s Ontological Argument

What is Ontology?

This argument is called the Ontological Argument for God’s existence because it deals with the ontology of God. Ontology deals with the nature or essence of a thing. There are four modalities of ontological existence, which can be condensed into three ontological categories of existence.

  1. The essence is able to exist and its existence can change.
  2. The essence is able to not exist and its existence can change.
  3. The essence is able to exist and its existence is not able to change.
  4. The essence is not able to exist and its existence is not able to change.

The two former modalities are both contingent, the third is necessary, and the fourth is impossible. These categories are all mutually exclusive and logically exhaustive, which means there is nothing that cannot fit into one of these categories, and there is nothing in more than one of these categories.

What is the Ontological Argument?

There are several different formulations for the Ontological arguments. The first is as it was originally proposed by St. Anselm is a little different than some other proofs for God. it is formulated as such:

Hence, even the fool is convinced that something exists in the understanding, at least, than which nothing greater can be conceived. For, when he hears of this, he understands it. And whatever is understood, exists in the understanding. And assuredly that, than which nothing greater can be conceived, cannot exist in the understanding alone.
For, suppose it exists in the understanding alone: then it can be conceived to exist in reality; which is greater.

St. Anselm, Proslogion, Ch. II

In other words, if we can imagine something “which greater nothing can be conceived” it must also exist in reality, or else it would not be the greatest being that can be conceived.

There is one thing about this argument that is slightly different than some other arguments for the existence of God. This argument is an argument a priori. If something is a priori, it means that it is not based on scientific observations. For example, the fine-tuning argument is an argument a posteriori. You can make the observation that our world is fine-tuned and you extrapolate out. A priori means you do not extrapolate based on scientific observations, but by the nature of things. So I could say that there are no married bachelors not because I have talked to every bachelor (which would be a posteriori) but because I know that the definition of a bachelor is one who is unmarried. So in the case of the ontological argument, we are extrapolating from the definition of God to prove He truly does and must exist.

Alvin Plantinga’s Version

Since St. Anselm, there have been many other formulations that work off the basic definition of God. The most known, other than St. Anselm’s original, has been a formulation by the renowned Christian philosopher Alvin Plantinga. His formulation works off of the three ontological categories of existence. If we look at these categories, we can take a look at the necessary category of existence. The necessary category is a being that does exist and its ontological status cannot be changed. So any being that is in this category (God) must exist, or it is impossible for it to exist. In other words, it is logically necessary for this being to exist. The only thing that could make this being not exist is if it were logically impossible. And as there is no logical impossibility in God’s nature, He must exist. If it is possible for Him to exist, He must exist.

But why does a necessary being need to be like the Christian God? Why can’t there be a necessary rock? As explained earlier in the article, there are three ontological categories of existence. There is nothing that can exist in more than one of these categories because there is nothing whose existential status both is able and not able to change, and there is nothing that does and does not exist at the same time. In addition, there is nothing that is not in one of these categories. This also extends to certain qualities. Because purpleness per se exists in the contingent category, it cannot exist in the necessary category. This goes for every other attribute: size, mass, colour, etc. Because of this, size, mass, and colour cannot be in the necessary category. Everything in the contingent category has some limiting attribute, and as these cannot exist. There can be no limitation in the necessary category. As there is nothing perfect and simple in the contingent category, the necessary category must be perfect and simple.

Guanilo’s Objection

A Benedictine monk named Guanilo responded to St. Anselm’s argument by asking why we could not posit a necessary island. If St. Anselm is simply positing that God has to exist, why can Guanilo not just posit a necessary island? This objection does not work for one main reason. Islands have strict limitations. They are confined to a certain geographical location, have a certain shape, and have certain colors. These cannot exist in the necessary category. Size, geographical location, and color are contingent attributes, so they cannot be part of something that is necessary.

Why Not More Than One?

An objection to this argument is that this leads to more than one necessary being. We wouldn’t have the Christian God, we would have some sort of polytheism. But this objection does not work. If there were to be two necessary beings, the second one would exist for the same reason as the first one. It would need to exist because it would be necessary. So what would be stopping this from continuing ad infinitum? If a second necessary being were to exist, then so would a third, fourth, fifth, etc. If you were to allow more than one necessary being, you would need to explain why this would stop at any specific number. There needs to be at least one, but there is no reason there would be more than one, and there is a reason to believe there would be only one.

There is also something quite curious about this objection. It seems that if you were to allow for the possibility of more than one necessary being, we would still have one, simple God. The two ways that things can differ are through quality and number. A key and a pencil differ in quality because they have different shapes, sizes, and colours. They also differ in number because there are two distinct and distinguishable objects. If we were to have more than one necessary being, they would not differ in quality, because they would all be necessary and without any limitations. And they could not differ in number, because this would make the state-of-affairs necessary, which would make the necessary beings logically prior to the state of affairs, which would make the state-of-affairs reliant upon the beings. So if the state of affairs is necessary, it has to be logically prior to everything else, which it would not be if there were more than one necessary being, because they would comprise the state-of-affairs. And if the state of affairs is necessary, it must be simple. Because simplicity does not exist in the contingent category, it must exist in the necessary category.

The Problem of Evil

One of the objections used to prove a logical inconsistency in God is the problem of evil. They say that if God is all powerful and all good, He could only do what is perfect. Either He’s all-powerful and evil, or He’s incompetent and all-good. This objection does not work for one main reason. It is impossible for us to determine if this objection works, because it is logically possible for God to bring more good than bad out of an apparently bad situation. If the atheist cannot prove that God is incapable of bringing good from evil, this objection does not work.


This argument is quite a strange one. It seems almost gimmicky. If God can exist, than He must exist. It seems like we are simply defining God as a being that must exist. But as we can see, there is good reason to believe that this argument does in fact work, in at least one of its formulations.

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