The ontological argument for the existence of God was first proposed by the medieval philosopher St. Anselm. Ontology as a philosophical sub-discipline is concerned with the nature of being. While Anselm himself did not propose an ontological system he was very much concerned with the nature of being. He argued that there are necessary beings -- things which can not not exist -- and contingent beings -- things that can not exist. The ontological argument for the existence of God in all of its interpretations and forms ends with a statement like "God exists and is a necessary being". A very colloquial version of Anselm's conclusion is "God can't not exist." This is obviously a controversial position, and the ontological argument has a long history of detractors and defenders.

Anselm's Original Argument

Anselm presents the ontological argument as part of a prayer directed to God. He starts with a definition of God, or a necessary assumption about the nature of God, or perhaps both.

"Now we believe that [the Lord] is something than which nothing greater can be imagined."

Then Anselm asks the big question - does God exist?

"Then is there no such nature, since the fool has said in his heart: God is not?"

To answer this, first he tries to show that God exists 'in the understanding':

"But certainly this same fool, when he hears this very thing that I am saying - something than which nothing greater can be imagined - understands what he hears; and what he understands is in his understanding, even if he does not understand that it is. For it is one thing for a thing to be in the understanding and another to understand that a thing is."

Anselm goes on to justify his assumption, using the analogy of a painter:

"For when a painter imagines beforehand what he is going to make, he has in his undertanding what he has not yet made but he does not yet understand that it is. But when he has already painted it, he both has in his understanding what he has already painted and understands that it is.

Therefore even the fool is bound to agree that there is at least in the understanding something than which nothing greater can be imagined, because when he hears this he understands it, and whatever is understood is in the understanding."

Now Anselm introduces another assumption:

"And certainly that than which a greater cannot be imagined cannot be in the understanding alone. For if it is at least in the understanding alone, it can be imagined to be in reality too, which is greater."

Example: Most people would prefer a real £100 as opposed to an imaginary £100

"Therefore if that than which a greater cannot be imagined is in the understanding alone, that very thing than which a greater cannot be imagined is something than which a greater can be imagined. But certainly this cannot be."

Anselm's found a contradiction! From that contradiction, he draws his conclusion:

"There exists, therefore, beyond doubt something than which a greater cannot be imagined, both in the understanding and in reality."

Perfections

In order to understand the place this argument has in the history of philosophy, it is important to understand the essence of the argument as Anselm first conceived it.

A key to understand the ontological argument is understanding the idea of "perfections."

There are various kinds of so-called perfections. Size, intelligence, beauty, power, benevolence, and so forth -- all these qualities are called perfections. And there are various degrees of these perfections. What is more intelligent is more perfect as regards intelligence; what is more beautiful is more perfect as regards beauty; and so forth.

Here's a short, and very general description of the ontological argument.

Anselm, and many others, define God as the perfect being. This means that he has all the perfections to the greatest conceivable degree. Indeed it seems that all of the monotheistic religions make the claim that there is nothing greater than God.

In fact, Anselm would say if someone were ever to conceive of some greater degree of perfection, than he or she previously conceived of, then they must regard God as having that greater degree of perfection.

According to Anselm this proves that God exists, since claiming that God is only imaginary entails a logical contradiction. To see why this is so take a look at these two suppositions.

1) Someone has a concept of the greatest conceivable being, which they call God.

2) This concept of God as the greatest conceivable being exists only in his or her mind.

St. Anselm says that these two suppositions contradict each other because for both 1 and 2 to be true, the following must also be true:

A) It is possible to have an idea of the greatest conceivable being.

B) And that this most perfect conceivable being is just imaginary.

But in that case the idea of God is as a being that is only imaginary; so it is not a concept of the greatest conceivable being, because a being that actually exists is a greater being, more perfect, than a being that merely exists in only in the imagination. Existence is a perfection, just like power, beauty, and so forth. So, the concept of the greatest conceivable being, must be a concept of a being that actually exists. A being that didn't exist wouldn't be the greatest conceivable being.

That means that the second supposition, that the concept of God exists only in our minds, contradicts the first supposition, that we have a concept of God.

This leads to Anselm's conclusion that if someone has a concept of God, then God necessarily exists. Since, it is impossible to deny the first supposition, because lots of people do seem to have a concept of God as the most perfect being, the only solution is to throw out 2.

One of the earliest recorded objections to Anselm's argument is was raised by one of Anselm's contemporaries, Gaunilo. Gaunilo invited his readers to think of the greatest, or most perfect, conceivable island. As a matter of fact, no it is likely that no such island actually exists. However, Guanilo's argument would then say that we aren't thinking of the greatest conceivable island, because the greatest conceivable island would exist, as well as having all those other desirable properties. Since we can conceive of this greatest or most perfect conceivable island, then it must exist. While this argument seems absurd, Gaunilo claims that it is no worse than Anselm's.

Defenders of Anselm's argument answered that the idea of an island does not include the notion of perfection, the perfection is merely tacked on, while the concept of God cannot be separated from the notion of perfection. This explains their claim that there is an explanation for the failure of Gaunilo's argument – namely the fact that the island's perfection is contingent -- which doesn't effect the Ontological Argument.

Another traditional criticism of the argument is that existence is not a perfection, because existence is not a property. If existence were a property of items, including God, then it might be a perfection. But since it's not a property, it cannot be a perfection; only properties of things can be perfections of things. But it is hard to understand how existence could not be a property, and many philosophers have rejected this objection because it entails all kinds of other unwanted consequences.

A third criticism of Anselm's argument rests on the claim that, even if existence is a property, it is still not a perfection. Either something exists or it doesn't; there aren't any degrees of existence. Something can't be more or less existent. It is claimed that this is sufficient reason to believe that existence can't be a perfection because perfections exist along a continuous scale. Defenders of the ontological argument have replied to this objection that, even granted that there aren't degrees of existence, surely existence is more perfect than nonexistence; and in any case, something that exists is greater than something that doesn't.

Obviously Anselm thought this argument was valid and persuasive, and it still has occasional defenders, but many, perhaps most, contemporary philosophers believe that that the ontological argument, at least as Anselm articulated it, does not stand up to strict logical scrutiny.

Some of those who've argued that the ontological argument fails are content to leave it at that, either because they don't believe that God exists, or because they believe the existence of God is demonstrated on other grounds. Others, like Alvin Plantinga, Kurt Gödel, Charles Hartshorne[?], and Gottfried Leibniz have reformulated the argument in an attempt to revive it.