In this case, the possibility must BE the reality, except that you are too blind and stupid to see it. So tell me, How DO things start by themselves? You who seem to think that shit can just form on its own, with absolutely no viable explanation as to why...
Its really a very simple argument, Shit for Brains.
A. I can conceive of a Being a greater than which cannot possibly be conceived.
B. Existence is greater than than nonexistence.
C. Ergo, G-d exists.
Trying to argue, as Guanilo did, that the argument fails based on the "great island" example fails in and of itself. An island is not necessarily great due to existence. In fact, if I proposed an island to exist in the middle of an inappropriate place, the results could be disastrous.
I have even heard someone propose that a greatest ham sandwich disproves the argument. Not at all. What if it is too big to eat, and there is no one with whom to share it, and no way to refrigerate the leftovers?
G-d clearly does not have either of these problems. But clearly, if I can conceive of him, he must exist, else the world would be rendered less logical.
Clearly the argument doesn't stand alone. That would be stupid. It stands together with other inductively strong arguments in favour of the existence of G-d.
And do realise that I never claimed that the existence of G-d could be deductively proved. It can be inductively inferred with very strong arguments. It cannot be deductively proved, unless someone is able to go to G-d's residence and come back with verifiable proof that he has been there, met the Almighty, and come back. So far that hasn't happened.
But denial of G-d is just straight out stupid. The atheist mindset is so fucked up its unbelievable to anyone with a brain. To claim that you know that G-d does not exist just makes you a complete 'Tard of the Eff variety. The result of atheism in human history has been destruction and death on a level unparalleled by any other ideology that has ever existed. The atheist sets himself up as the the master of all he surveys, and the result is disaster.