The real question is: Is there anything we can think of which, by the mere fact that we can think of it, is shown to exist outside our thought? Every philosopher would like to say yes, because a philosopher's job is to find out things about the world by thinking rather than observing. If yes is the right answer, there is a bridge from pure thought to things. If not, not.
My own feeling, to the contrary, would have been an automatic, deep suspicion of any line of reasoning that reached such a significant conclusion without feeding in a single piece of data from the real world. Perhaps that indicates no more than that I am a scientist rather than a philosopher. Philosophers down the centuries have indeed taken the ontological argument seriously, both for and against. The atheist philosopher J. L. Mackie gives a particularly clear discussion in The Miracle of Theism. I mean it as a compliment when I say that you could almost define a philosopher as someone who won't take common sense for an answer.
The most definitive refutations of the ontological argument are usually attributed to the philosophers David Hume (1711-76) and Immanuel Kant (1724–1804). Kant identified the trick card up Anselm's sleeve as his slippery assumption that 'existence' is more 'perfect' than non-existence. The American philosopher Norman Malcolm put it like this: 'The doctrine that existence is a perfection is remarkably queer. It makes sense and is true to say that my future house will be a better one if it is insulated than if it is not insulated; but what could it mean to say that it will be a better house if it exists than if it does not?'46 Another philosopher, the Australian Douglas Gasking, made the point with his ironic 'proof that God does not exist (Anselm's contemporary Gaunilo had suggested a somewhat similar reductio).
Ergo:
Needless to say, Gasking didn't really prove that God does not exist. By the same token, Anselm didn't prove that he does. The only difference is, Gasking was being funny on purpose. As he realized, the existence or non-existence of God is too big a question to be decided by 'dialectical prestidigitation'. And I don't think the slippery use of existence as an indicator of perfection is the worst of the argument's problems. I've forgotten the details, but I once piqued a gathering of theologians and philosophers by adapting the ontological argument to prove that pigs can fly. They felt the need to resort to Modal Logic to prove that I was wrong.
The ontological argument, like all a priori arguments for the existence of God, reminds me of the old man in Aldous Huxley's Point Counter Point who discovered a mathematical proof of the existence of God:
You know the formula, m over nought equals infinity, m being any positive number? Well, why not reduce the equation to a simpler form by multiplying both sides by nought. In which case you have m equals infinity times nought. That is to say that a positive number is the product of zero and infinity. Doesn't that demonstrate the creation of the universe by an infinite power out of nothing? Doesn't it?
Or there is the notorious eighteenth-century debate on the existence of God, staged by Catherine the Great between Euler, the Swiss mathematician, and Diderot, the great encyclopedist of the Enlightenment. The pious Euler advanced upon the atheistic Diderot and, in tones of the utmost conviction, delivered his challenge: 'Monsieur, (a + bn)/n = x, therefore God exists. Reply!' Diderot was cowed into withdrawal, and one version of the story has him withdrawing all the way back to France.
Euler was employing what might be called the Argument from Blinding with Science (in this case mathematics). David Mills, in Atheist Universe, transcribes a radio interview of himself by a religious spokesman, who invoked the Law of Conservation of Mass-Energy in a weirdly ineffectual attempt to blind with science: 'Since we're all composed of matter and energy, doesn't that scientific principle lend credibility to a belief in eternal life?' Mills replied more patiently and politely than I would have, for what the interviewer was saying, translated into English, was no more than: 'When we die, none of the atoms of our body (and none of the energy) are lost. Therefore we are immortal.'
Even I, with my long experience, have never encountered wishful thinking as silly as that. I have, however, met many of the wonderful 'proofs' collected at http://www.godlessgeeks.com/LINKS/GodProof.htm, a richly comic numbered list of 'Over Three Hundred Proofs of God's Existence'. Here's a hilarious half-dozen, beginning with Proof Number 36.
36 Argument from Incomplete Devastation: A plane crashed killing 143 passengers and crew. But one child survived with only third-degree burns. Therefore God exists.
37 Argument from Possible Worlds: If things had been different, then things would be different. That would be bad. Therefore God exists.
38 Argument from Sheer Will: I do believe in God! I do believe in God! I do I do I do. I do believe in God! Therefore God exists.
39 Argument from Non-belief: The majority of the world's population are non-believers in Christianity. This is just what Satan intended. Therefore God exists.
40 Argument from Post-Death Experience: Person X died an atheist. He now realizes his mistake. Therefore God exists.
41 Argument from Emotional Blackmail: God loves you. How could you be so heartless as not to believe in him? Therefore God exists.
THE ARGUMENT FROM BEAUTY
Another character in the Aldous Huxley novel just mentioned proved the existence of God by playing Beethoven's string quartet no. 15 in A minor ('heiliger Dankgesang') on a gramophone. Unconvincing as that sounds, it does represent a popular strand of argument. I have given up counting the number of times I receive the more or less truculent challenge: 'How do you account for Shakespeare, then?' (Substitute Schubert, Michelangelo, etc. to taste.) The argument will be so familiar, I needn't document it further. But the logic behind it is never spelled out, and the more you think about it the more vacuous you realize it to be. Obviously Beethoven's late quartets are sublime. So are Shakespeare's sonnets. They are sublime if God is there and they are sublime if he isn't. They do not prove the existence of God; they prove the existence of Beethoven and of Shakespeare. A great conductor is credited with saying: 'If you have Mozart to listen to, why would you need God?'