3) "Therefore God must exist" - this is the logical fallacy, the "base assertion" fallacy I believe it's called. The only thing you can actually prove with the above two things would be something like "Therefore, the definition of God would include him actually existing." The only reason you'd conclude that he ACTUALLY exists is if you assumed that your definition at the beginning was describing something that actually exists, and that's begging the question. For example, if I said "The definition of 'Dream Angelina Jolie' is that she's a clone of Angelina Jolie but she is also my girlfriend." and "In order to be my girlfriend, she'd have to exist", it (unfortunately) does not prove "Therefore, Dream Angelina Jolie exists and is my girlfriend." The only thing it proves is "Therefore, the definition of Dream Angelina Jolie would have to include existence as one of her traits".
Thats why I said the following in my post:
Note: You must agree with the definition of God above if this theory is to be valid. Most people who agree that is the definition of God.
"That which than nothing greater" - the word "greater" is a loaded term. What's great for you may not be great for me. In fact, I'd say the word "great" is so vague as to be almost meaningless. When I imagine the "greatest" sandwich that can be conceived of, it's probably very different than the "greatest" sandwich you would imagine.
Yes its a hard word to define, buts its meaning is really quite accepted by everyone. For example, we all know what Hate and Love is, but how can we define it? Just because its hard to define it doesnt make it invalid. Great can be subjective in certain circumanstances (e.g. whats the greatest sandwhich) but in the Onotlogical argument context, its not really.
1) Imagine God. - It's not actually possible to imagine God, since no one has any idea what they're talking about when they use that word and there's no actual definitive version of God, and no way to know if what you're imagining is God or not. I could imagine a ham sandwich and claim that I'm imagining God. But is that true? How do I know that what I'm imagining is the "greatest" thing, or even great at all? It's clear that Frosted Flakes are grrreat, but with other things, their level of greatness is questionable.
You can "imagine God". You just cant imagine A LOT of his qualities (if not all) as they are out of our world/experience. I can tell you now that I am imagining him now - for a fact. Although I cant actually know for sure a lot of things about him, i can still "imagine" him.
Still, if you disagree with my points above, just replace the step "Imagine God" with "Imagine Something" and "God exists in the mind only" and "Something exists in the mind only" and the argument will still work.
2) "It's greater to exist physically than just in the mind" - says who? Again, this is opinion and usage of the weasel-word "great". But let's say we accept this argument; it doesn't really matter that much.
Would you rather exist in the mind only, or in the mind and reality?
And I never said "it's greater to exist physically than just in the mind". I said "its greater to exist physically AND in the mind than just the mind". Its generally accepted that this is the case, however the majority of people would agree that its greater to exist in both than just one. And with God specifically, a God that existed in the mind and reality would be superior and have more control than the God that just existed in the mind (which is the point of the argument).