What do you think of this argument for God?

[Angrboda]

Yeah, I've heard this one before. I don't accept the first two premises to be true. The third premise is a guess that God exists in any possible world in the first place, so I can't accept that to be true either. The rest crumbles with that flawed foundation.

The first premise is true if the theist defines God that way (after all, you can't refute a definition unless it is logically inconsistent). The second just means that since God is necessary (must exist), the existence of God implies existing in every possible world (logically impossible to not exist).

The first premise is incoherent if by greatest one means objectively greatest. Greatness is always the function of a value system. Something is great according to some set of criteria. There are no properties which are great in and of themselves. (And if by greatest one means subjectively, then the whole argument collapses.)

[WinterHold]

I would rather follow "the Argument from Design" for the existence of God; there's no magical spell for an argument; God's signs are just enough proof for him.

He would exist outside the context of time and space, also inside it.

[SuperSentient]

And that's exactly why I was mostly aiming the argument about possibilities at the rest of the premises. To bring all that back to the first premise, what if the greatest possible being turns out to be nothing more than human? Would you call that being "God" by your definition? If so, then I don't see the point here.

If the greatest possible being turned out to be human, than that being would be God.

1. God is the greatest possible being. 
2. The greatest possible being turns out to be a human.
3. Therefore, God is human(s)

What points to this as flawed is this:
-What defines "greatness"? There must first be an objective standard for determining greatness. If you have one, is a cat greater than a dog by this standard?
-How do you know the greatest possible being is a necessary being? Why can't I say the Flying Spaghetti Monster is a necessary being as well then if it is possible for this monster to exist, it must exist in all possible worlds.

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I would rather follow "the Argument from Design" for the existence of God; there's no magical spell for an argument; God's signs are just enough proof for him.

He would exist outside the context of time and space, also inside it.

You mean the teleological argument?

[Jesster]

If the greatest possible being turned out to be human, than that being would be God.

1. God is the greatest possible being. 
2. The greatest possible being turns out to be a human.
3. Therefore, God is human(s)

Then we agree on that conclusion based on the premise. I just don't see the point of that label at that point. I would just call that "human". This is much like how you said we already have a universal definition for "apple" before.

What points to this as flawed is this:
-What defines "greatness"? There must first be an objective standard for determining greatness. If you have one, is a cat greater than a dog by this standard?
-How do you know the greatest possible being is a necessary being? Why can't I say the Flying Spaghetti Monster is a necessary being as well then if it is possible for this monster to exist, it must exist in all possible worlds.

I was asking the same question about what defines "greatness" earlier. I don't see it as a measurable trait. So far it seems a bit subjective.

[Simon Moon]

I heard an argument like this:

1. God is the greatest possible being.

2. God is a necessary being, which means that God exists in every possible world (If God exists). 

3. If God exists in one possible world, God must logically exist in every possible world. 

4. Since God is the greatest possible being, it follows that every aspect of God (being possible) exists in some possible world.

5. Therefore, God exists (in all possible worlds, including ours).

I actually just structured the premises this way myself but is the same idea as an argument I heard before. 

What do you think of it?

Just another variation of an ontological argument.

Sorry, but you can't define a god into existence.

Basically what this argument does is list some attributes that this god would have, then add one more attribute, which is existence. But existence is not an attribute. To have attributes, the thing under discussion exist in the first place.

[SuperSentient]

I heard an argument like this:

1. God is the greatest possible being.

2. God is a necessary being, which means that God exists in every possible world (If God exists). 

3. If God exists in one possible world, God must logically exist in every possible world. 

4. Since God is the greatest possible being, it follows that every aspect of God (being possible) exists in some possible world.

5. Therefore, God exists (in all possible worlds, including ours).

I actually just structured the premises this way myself but is the same idea as an argument I heard before. 

What do you think of it?

Just another variation of an ontological argument.

Sorry, but you can't define a god into existence.

Basically what this argument does is list some attributes that this god would have, then add one more attribute, which is existence. But existence is not an attribute. To have attributes, the thing under discussion exist in the first place.

That is the major fallacy of all variations of the ontological argument. They assume God is logically necessary and go from there.

[Faith No More]

This being would be independent of what we know, so it wouldn't matter what we all know is possible, just that this being is the greatest being possible. 

"Greatest" would be a manifestation of every logical possibility.

Surely the "greatest possible being" is one that could do the impossible, no? If it can create the laws of logic, why can't it violate them?

I think the greatest possible being is one that could create the universe without ever having existed in the first place, but that's just what you get when you toss around subjective terms like they have objective meaning.

[SuperSentient]

This being would be independent of what we know, so it wouldn't matter what we all know is possible, just that this being is the greatest being possible. 

"Greatest" would be a manifestation of every logical possibility.

Surely the "greatest possible being" is one that could do the impossible, no?  If it can create the laws of logic, why can't it violate them?

No, it couldn't do the impossible by definition. It wouldn't create "laws of logic", which would just be essential unchanging propositions. 

I think the greatest possible being is one that could create the universe without ever having existed in the first place, but that's just what you get when you toss around subjective terms like they have objective meaning.

A non-existent being creating the universe? 
Yes, being "greatest" would be subjective.

[Nonpareil]

That is the major fallacy of all variations of the ontological argument. They assume God is logically necessary and go from there.

Well, not exactly. They try to assert that God is logically necessary by defining the term "God" as "a being that is logically necessary".

Unfortunately, even if we were to grant that the definition is coherent - and it usually isn't - merely defining something as necessary does not establish that there exists an actual instance of that object. You still have to show that there is an actual logical necessity. I can define unicorns as logically necessary, but that doesn't mean that they actually are.

It's just bare assertion which quickly becomes circular logic, because theistic philosophers, as a rule, are very bad at their jobs.

[SuperSentient]

That is the major fallacy of all variations of the ontological argument. They assume God is logically necessary and go from there.

Well, not exactly. They try to assert that God is logically necessary by defining the term "God" as "a being that is logically necessary".

They do in the case of the ontological argument, especially this variety:

1. If it is possible for God to exist, then God exists in every possible world.

2. It is possible for God to exist.

3. Therefore, God exists in every possible world.

With 3, it follows that God exists. The reason that the possibility of God means the existence of God is that God exists in every possible world as a necessary being. I don't see theists trying to justify the conception of God being necessary, they pretty much assume it, just as they assume that God is omnipotent. It is basically part of the concept of God. 
Theists may argue that since all contingent things must have an explanation and that explanation at some point should be necessary (exists with no explanation), then they just claim that they call this necessary explanation God.