Why Plantinga's modal ontological argument fails

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First the argument:
http://en.wikipedia.org/wiki/Ontologica ... modal_form

I shall just call the proposition 'omniscient, omnipotent and perfectly good being exists' G.

Consider the definition of the term 'possibly'. In view of the comments in Wiki, the term 'possibly X' means
a) X has not been disproved.

It is clear that with this definition, P4) does not follow from P3) i.e. we have not disproved (necessarily G) does not imply (necessarily G) is true.

On the other hand, in the possible worlds semantics,
http://plato.stanford.edu/entries/logic-modal/
the statement possibly necessarily G means:
b) the statement 'G is true in all worlds in W' is true in some world W'.
In other words, to apply the S5 modal logic in the many world semantics, one has to establish 'G is true in all worlds in W' in some world and not just appeal to ignorance. Both P4) and P5) fails under interpretation b).

To conclude, saying that the S5 modal logic applies simply because the same words 'possibly' and 'necessary' are used is just a fallacy of equivocation.

Last edited by 01001011 on 05 Apr 2011, 8:10 am, edited 1 time in total.

Word Salad.

It is a rehash of Anselm's bogus arguement.

ruveyn

Ok, the issue is that "possibly X" usually really is best understood as "X is logically possible" not that "X has not been disproved"

In any case, P6 DOES follow from P5. That part is indisputable. If something is necessarily true, then it is true. The real issue you are getting at is the P4) and P5) issue, and that relies on "possibly it is necessarily true". Now, this isn't problematic in the modal logic, as such, an absurdity there would be an absurdity in other modal logic efforts.(Please note: such an absurdity is arguable)

The point you make against "true in some worlds" is that this isn't an appeal to ignorance. The entire argument is that if God is logically possible, then God must be actual. If God is logically possible as a necessarily existing being, then he must exist as a necessarily existing being. Given that "possible worlds" really refers to logically possible worlds, all that needs to be established is a lack of contradiction for a possible world argument.

Look, if you want to attack the ontological argument, you would be better off attacking the indeterminacy in the first premise using Gaunillo's island objections, as Plantinga's definition of "maximal excellence" is wholly arbitrary to the logical structure of the argument. Otherwise, you are left trying to argue modal logic, which... unless you understand modal logic well, is probably a waste of time.

Define 'logically possible' and 'lack of contradiction for a possible world'. The term Possibly has a technical definition depending on the exact structure of the modal logic being used, built on a formal Kripke semantics. http://en.wikipedia.org/wiki/Kripke_semantics and is not interchangeable with the everyday word "possibly" It appears you don't understand modal logic very well.

Logically possible? Well, that means "lack of a contradiction in terms" http://en.wikipedia.org/wiki/Logically_possible Also, "lack of contradiction for a possible world" is not a technical term at all, and at best is actually two terms put together. What I was referring to is the dependence on logical possibility for possible world frameworks, so if something lacks contradiction, thus is not necessarily not the case, then it is a possible world.

In any case, the Kripke definition of "possibly" given in wikipedia means "not necessarily not" something. If something isn't false by necessity, then it is possible, most things false by necessity are logically false/impossible.

In any case, Binary, you haven't really disproven anything I said in my counter to you. P6 really does follow from P5, as things necessarily true, are true, because they cannot be false. Period. End of statement. You may have made a typo or something. Even further, for you to say that I must lack knowledge of modal logic, when your own definition of "possibly" is "not been disproven" is very questionable. No, possibly means "without contradiction". Something could be impossible, and we not know it, such as a contradiction we didn't detect, and if that is the case, it is necessarily true that this thing is not possible. In fact, one criticism of the ontological argument is that we do not have the epistemic power to determine whether or not God is possible.

In any case, once again, the matter of "possibly" means that Plantinga really only has to assert a lack of logical contradiction in order to invoke the modal possibility, as if there is a lack of contradiction, then his invocation is fine. I don't see the appeal to ignorance, and what you seem to require of Plantinga to make the argument in many world semantics doesn't make sense. How do you establish "G is true in all worlds in W" in some world, other than to ask whether it is logically coherent, and if one presumes that it is logically coherent, the issue works just fine.

Finally, in any case, regardless of whether anything I am saying is right. Do you really, I mean *really* think that trying to attack Plantinga's modal logic will get your point across? Most people, including theists, don't know modal logic. Given that Plantinga's argument still is considered as standing by a large number of people, either philosophers don't know modal logic, or your points aren't as basic as you think. As such, why go this route instead of a Gaunillo route?

The argument assumes that Perfection is more than an ephemeral, subjective human preference, and that perfection is some objective quality like mass or speed.

The argument is nonsense. As Dawkins said, if the argument assumes a Maximally "X" being exists for any trait X (why would perfection be special), there should be a maximally reeking, smelly being, who would stink more if he existed.

Or maybe a maximally large being (which is an objective quality), who would clog up every inch of the Universe with his fat clogged ass.

I don't see how this passes for logic.

EDIT, should read P4) does not follow from P3).

Your former paragraph seems to be getting at it but your later paragraph contradicts the latter. Let me give you some hints.

Say, what does it mean by 'it is logically possible that the 1 zillionth digit of pi (necessarily) equals 1'? There is no logical contradiction it and only it the 1 zillionth digit of pi is actually 1. In other words to assert that ' it is logically possible that the 1 zillionth digit of pi equals 1' in the model sense, one has to compute the 1 zillionth digit of pi, rather than appeal to ignorance to any known contradiction.

That is what S5 is saying: the truth value of the statement 'necessarily X' is independent of the world where the statement is evaluated. One may use the axiom only if P4 is true in the (much stronger) modal sense. On the other hand, it is obvious that, when understood in the modal sense, P4 (or P3, depending on how one interprets it) is just a massive baseless assertion that begs the question.

As for your last question, Plantinga (can he be considered philosopher?) tries to use S5 modal logic, there is no reason why I cannot attack his misuse. For those who don't understand modal logic, they are in no position to support Plantinga. My argument directly address the issue and is almost impossible to dodge.

It means nothing. Pi, as defined in its geometric properties, has a logically required value. It may be weakly conceivable, but the actual value of pi is necessarily the case.


The problem with pi is known. The problem with God is not known. Even further though, the argument basically hinges on P3, where yes, if you know what is entailed, you know that either God must exist, or he must not, but.... that doesn't stop the argument from being used. A lot of people who try to use it seem to "beg the doxastic question" in that they see God belief as more natural than the lack, thus we should presume possibility more than impossibility.


Plantinga is widely considered a philosopher. Nobody I know of does not consider him a philosopher.

Even further, nobody is going to change their use of the argument on modal logic. In fact, I'd suggest that only a minority have heard of it, and most are not equipped to deal with it sensibly, even if they do want to quote this argument. When defeating a position, it is best to work with elements that are clear. Nobody with a strong opinion will defer to an argument they don't quite understand. (and to be fair, nobody really cares about ontological arguments anyway, as they aren't that compelling to most people)

Then think of something like the Riemann hypothesis. What do you mean by 'it is logically possible that the Riemann hypothesis is true? What about 'it is modal possible that the Riemann hypothesis is true?



Are you really falling for this trick? By definition, the only way to justify the claim 'it is modal possible that God exists' is to establish 'God exists' in some world. If such being is defined to be maximally great in all worlds, then the statement has to be checked for all worlds.

The theist belief at best justify the claim 'there is no reason to reject the existence of God'. That is much weaker than the modal claim and is irrelevant to the modal argument.

Those who quotes the modal argument but not understand modal logic are pwned from the beginning.

Of course, part of the problem is that in using any mathematical analog, you're dealing with things that are not ever contingently true. God's existence is imaginable as contingently true, which leads to the odd idea of trying to use these things.


No, it has to avoid contradiction. That's just the nature of possible worlds arguments. Math, because it exists in a framework, has a pre-set matter of whether it coheres or not. God, is just a random fact though, and one conceivable(without taking into account necessity) as such enough to hold it as true enough to make the argument.

In any case, no, I don't think "the statement has to be checked for all worlds". The statement has to meet some level of examination before we can buy into the argument, yes. But, that level of examination is simply the avoidance of incoherence.


The issue is that to move from "there is no reason to reject the existence of God" to "God is possible" is valid. Thus putting us on the same track.


So? If you can't explain to them where they are wrong, (which is difficult enough using just *regular* logic) then what's the point? It isn't as if any victory gotten would be meaningful given that.

The Riemann hypothesis is either true or false. If it is true, then it is possible but if it is false it is impossible. The Riemann hypothesis either follows from the axioms for complex numbers or it does not. There is no modality involved other than true or false.

ruveyn

@AG
Read the argument again. It is trying to prove that God _necessarily_ exists. i.e. a non-contingent fact.

So when you say it is possible God necessarily exists, how to you guarantee there is no inconsistency?