The Ontological Argument - valid or debunked?

[GrandizerII]

2 is just simply true (as true as "all bachelors are unmarried")

No, "all bachelors are unmarried" is a logically valid tautology. The following is a logically invalid non-sequitur:

Premise 2: If it's possible that a maximally great being exists, then a maximally great being exists in some possible world.

If something is logically possible it does not at all entail that that something exists in any world.

and 3 is based on the argument that the maximally great being, as defined by Plantinga, is possibly necessary.

No it's another non-sequitur.

Premise 3: If a maximally great being exists in some possible world, then it exists in every possible world.

If something exists in a possible world it does not at all entail that it exists in all possible worlds. If something is possibly necessary that does not at all entail that it necessarily exists.

Sorry, Alasdair, but this is not right at all. I'm just a layman like you, so you don't need to take my word for it, but feel free to ask an academic atheist philosopher if what you're saying here is true. The only premise that should be in debate here is premise 1.

Again, possible world is not the same as an actual world. Possible world can be actual or just a world that's conceivably possible.

Second, premise 3 is not a non-sequitur because it follows logically from the prior premises in the case of an MGB (because an MGB by definition has to be necessary according to Platinga and others; if it's not necessary, then it's not an MGB).

So ways to counter premise 1:

1) It has not been established that premise 1 is possibly true. Contrary to what Christians like to say, you still need to provide some leveraging weight to the argument. As it stands now, it's just a really empty argument, despite its cleverness.

2) A maximally great being (defined as an omnipotent and omniscient being) is logically incoherent. No matter how powerful a being can get, there is always something it cannot do. If there are things it cannot do (like creating a rock that it cannot lift or destroy), then its power is restricted in some way. If there is a restriction, then it falls short of perfectness, and therefore is not really the maximally great being defined by Plantinga. Therefore, such a being cannot possibly exist.

3) Even if we were to overlook the paradox of omnipotence, and a maximally great being is allowed to not be able to do everything that can possibly be done by an entity, it cannot do what's logically impossible for any entity to do. For example, it cannot create out of literal nothingness; it cannot timelessly create time or anything for the matter; it cannot timelessly create in a mindful/sentient manner; and, of course, it cannot logically create a "square-triangle". So if you look at it this way, a maximally great being, if it exists, must have eternally coexisted with the ultimate universe/reality and always in time. It could not have been the Creator.

4) It has not been adequately argued, as far as I know, that sentience is an indicator of greatness. Perhaps, sentience is merely an indicator of evolution of beings that actually change and progress. Why would a maximally great being need to be sentient?

5) In line with option 3 and 4, the MGB need not be the traditional theistic god. In fact, at best, it can be a pantheistic sort of god that isn't the Creator but rather the Co-existor.

6) The ultimate universe itself might be the MGB (or maximally great entity).