The ontological argument fails at logic, while trying to use logic. Any conclusion deriving from a faulty pretense can be discarded. This is described by the rules of validity.
http://en.wikipedia.org/wiki/Validity When boiled down, the ontological argument comes out "The ontological argument is true because the ontological argument is true!", which is a tautology.
http://en.wikipedia.org/wiki/Tautology_(logic)
This is shown by anselm's insistence (which others have shown up as being false already) that simply because a thing can be conceptualized, that thing must be real. He treats this as an axiom, and then performs the tautological proof using this axiom. Based on the rule of validity, his argument can be summarily discarded after a simple example showing that the axiom is false, as was done earlier in the thread, QED.
This is Anselm's argument:
1.It is a conceptual truth (or, so to speak, true by definition) that God is a being than which none greater can be imagined (that is, the greatest possible being that can be imagined).
2.God exists as an idea in the mind.
3.A being that exists as an idea in the mind and in reality is, other things being equal, greater than a being that exists only as an idea in the mind.
4.Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (that is, a greatest possible being that does exist).
5.But we cannot imagine something that is greater than God (for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined.)
6.Therefore, God exists.
The tautology happens at axiom 3 (emphasis mine). "A being that exists as an idea in the mind
and in reality is, other things being equal, greater than a being that exists only as an idea in the mind."
It asserts without qualification that god is real, and thus naturally-- concludes that god is real.
The alternative interpretation of that sentence produces an undefined state in which something both is and is not true simultaneously. This is what happens when you remove the dependent clause:
"A being that exists as an idea in the mind and in reality is greater than a being that exists only as an idea in the mind."
In order for this to be true, then "god" would have to be so great that it cannot be conceived of by a mind, because it is greater than that conception. However, it negates itself, by asserting that it's existence in the mind is true.
This statement is not satisfiable, and therefor not valid.
http://en.wikipedia.org/wiki/SatisfiabilityIn so many words, anselm is full of shit, and was drinking too much of the koolaid.
Edit:
The more "interesting" thing about Anselm's argument is that it (by logical definition), means that all conceptions of god are false, because the true god must transcend all conceptions-- Meaning, if god "does" exist, it exists in a fashion that cannot be properly conceived of, meaning any religious doctrine claiming infallible knowledge of this entity must likewise be false (in at least some capacity). (Knowledge being a wholly mental conception of the actual thing, which the actual thing MUST transcend to satisfy the axiom.)
Congratulations Anselm, you managed to figure out (hard) agnosticism, while failing to accept it as true.
The more rational christian would thus understand that claims of infallibility in a written document about an incomprehensible subject matter must therefor be false, and that any valid comparison between the written documentation and the actual thing being documented must have some degree of conceptual error as an inherent property, simply because the written documentation is comprehensible.
They may take this with aplomb, since a truly real-to-life document (about an incomprehensible subject) would be incomprehensible, and thus not useful to a worshiper. EG, "The bible may not be perfect, but it is as close as can be made possible, given the incompatibilities of mortal minds when confronted with the divine." They may therefor state that the bible is "perfect", in the sense that it is able to accomplish its intended function in the best way possible, even if absolute perfection is not attainable. "It's the best it can possibly be, and still be fit for the intended function." Etc.
This means that hard-nosed literal application of the bible will be functionally incorrect against the incomprehensible ideal god actually espouses.
However, this does nothing whatsoever to "Prove" the existence of this god.