As with all philosophical arguments for the existence of God, there are a number of versions of the ontological argument. I'll lay out a couple of them:
Ansel's ontological argument (the original)
Descartes' ontological argument
- 1. If I am thinking of the Greatest Being Thinkable, then I can think of no being greater
- 1a. If it is false that I can think of no being greater, it is false I am thinking of the Greatest Being Thinkable
- 2. Being is greater than not being
- 3. If the being I am thinking of does not exist, then it is false that I can think of no being greater.
- 4. If the being I am thinking of does not exist, then it is false that I am thinking of the Greatest Being ThinkableConclusion: If I am thinking of the Greatest Being Thinkable, then I am thinking of a being that exists
- Whatever I clearly and distinctly perceive to be contained in the idea of something is true of that thing.
- I clearly and distinctly perceive that necessary existence is contained in the idea of God.
- Therefore, God exists.
Alvin Plantinga's ontological argument
1. It is possible that a maximally great being exists.
2. If it is possible that a maximally great being exists, then a maximally great being exists in some possible world.
3. If a maximally great being exists in some possible world, then it exists in every possible world.
4. If a maximally great being exists in every possible world, then it exists in the actual world.
5. If a maximally great being exists in the actual world, then a maximally great being exists.
6. Therefore, a maximally great being exists.
Alright. If any of these seem kind of ridiculous, it's because they are. But Bertrand Russel famously remarked that it's easier to feel like it's a fallacious argument than to point out precisely where the fallacy lies. Well, allow me to do that.
The ontological argument conflates concepts with actual things. There is a difference, for example, between the concept of God, and God himself. Everyone agrees that the concept of God exists, but that's not the same thing as saying that God actually exists.
The ontological argument makes a few errors. Plantinga, for example, uses the term "maximally great". But what exactly is "maximally great" supposed to mean? "Great" is an arbitrarily defined term, and the term "maximally great" is no more meaningful than "maximally sexy" or "maximally cool". The concept of "possible worlds" is similarly silly. A "possible world" is simply something we can conjure up in our imaginations. But just because we can dream it doesn't mean it's actually possible. We aren't dreaming up actual worlds, just concepts of worlds.
But the fatal flaw of the ontological argument is the notion that existence itself is a "great-making property". In other words, if we can dream something up, it's greater if it actually exists than if it doesn't exist. But again, anything we conjure up is merely a concept that is, in certain cases, representative of a real thing. When I'm thinking of, say, a horse, my thoughts are not an actual horse; they are the concept of a horse.
The problem for the ontological argument is that existence is a pre-requisite for something to have actual properties, rather than just concepts of properties. A horse has certain properties that make it a horse. But if it didn't actually exist, it wouldn't have any of those properties; rather, we could conjure up imaginary concepts of properties that would comprise this imaginary creature called a horse. But that's all they'd be – concepts. Some will argue that God exists necessarily; to quote Peter S. Williams:
The goodness of existing per se is a great making property that admits a logical maximum in necessary existence. And although - as Hume and Kant pointed out - saying that something ‘exists’ does not add to the list of its properties, to say that something ‘exists necessarily’ does add to its list of properties.But of course you can't exist necessarily unless you already exist in the first place – in other words, someone making the ontological argument would have to prove that God exists before they can prove that he exists necessarily. To assume otherwise simply begs the question.
So the ontological argument is circular because existence is not a property – it's what you have to have before something can actually have properties. Otherwise, you're simply talking about concepts, not actual things. Hopefully that cuts through some of the fog of this colossally weird argument.