"Imagine the most perfect being possible. Anything that you can imagine that would make this being better, add it in. Omnipotent? Immortal? All knowing? Add them all in. Now, thinking of this great being, surely one characteristic that it must have is the property of existing. If it doesn't exist, it's not very great, is it? So it must exist. (If what you are imagining doesn't exist, you're imagining the wrong thing, a less-perfect thing.) So, let us call this great thing God. And so, we have proven that God exists."
The above is an informal form of the ontological argument. There are certainly more complex formulations (see the other writeups in this node), but I'd like to try making it simpler, so that you can see one logical fallacy that often (perhaps always?) sneaks into the argument. Next I'm going to give you a formal statement of the argument above:
1. God is not flawed in any way.
2. A flawless being must have all properties that are desirable for a being to have.
3. It is desirable to have the property of existing.
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4) Therefore, God exists.
That's certainly a simplified version of the ontological argument, but it gives you the basic idea, and it helps prepare you for my next simplification. It is an attempt to put the ontological argument in such simple terms that the logical flaw becomes obvious.
Umit's Ontological Argument
1) God, by definition, is a necessary being
2) By definition, a necessary being exists
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3) Therefore, God exists.
This argument assumes what it is attempting to prove, begging the question. The first premise states that 'God is', and the conclusion is also 'God is'. So let's fix that. The correct form should be:
1) If God exists, Then he is a necessary being.
2) By definition, a necessary being exists
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3) Therefore, If God exists, then he exists.
Not quite as impressive once you state your assumptions correctly. But then, this is an extremely simplified version of the ontological argument. The ontological argument is tricky, because you are assuming that the 'if' in 'if God exists' is actually the case; assume that God does exist. What qualities does E have? Well... obviously, since you're assuming that E exists, E does, but more than that, E must exist.
Compare this to a unicorn. Assume that a unicorn does exist. Now imagine; the unicorn exists, but while it exists (in this imaginary world of yours), you can still imagine (in this imaginary world of yours) that a unicorn might not exist. Unicorns are like oranges and Vikings -- it's easy to imagine them not existing. But when you imagine God, E seems to have as one of Ier properties that E must exist. Otherwise E wouldn't be truly be God.
It is comparatively easy to alternate between believing that all forms of the ontological argument are slight-of-hand tricks and believing that "Eureka!, it does work!" However, it is not unfair to say that the great majority of analytical philosophers do not believe that any form of the ontological thus far developed actually proves the existence of God. Some theologians would disagree.
And so it goes.