Well i have been having a discussion with a theist and he has just brought up the modal ontological argument.  I have now  done some research on modal logic and have sites marked to further educate myself on it, but i am still way to new to it to have any clue whether my argument holds water or not, and i would really apreciate some help.

the argument he used goes like this
"1. It is possible that a maximally great being exists.
2. If it is possible that a maximally great being exists, then a maximally great being exists in some possible world.
3. If a maximally great being exists in some possible world, then it exists in every possible world.
4. If a maximally great being exists in every possible world, then it exists in the actual world.
5. If a maximally great being exists in the actual world, then a maximally great being exists.
6. Therefore, a maximally great being exists."- him

to which, after very little research i replied with


 "Now the modal ontological argument is not proof of anything .The modal ontological argument applies a string of logic but fails to test the logic. If the same string of logic can be used to come to an opposite conclusion, then the logic has been disproven through contradiction.

1) It is possible that a maximally great being does not exist.
2) If it is possible that a maximally great being does not exist, then a maximally great being does not exist in some possible world.
3) If a maximally great being does not exist in some possible world, then a maximally great being does not exist in any possible world.
4) If a maximally great being does not exist in every possible world, a maximally great being does not exist in the actual world.
5) If a maximally great being does not exist in the actual world, then a maximally great being does not exist.
6) Therefore maximally great being does not exist.

This uses the exact same logic to reach an opposite conclusion that is at odds with the original conclusion. This proves that the logic of the MOA is completely faulty and as such, illogical."-my reply

To which he replied


 "The issue I hold  that you does not effectively deny the rationality of there existing God. Here I would re-enter the absence of evidence ,does not, = evidence of absence argument. Due to the nature of the atheist's presuppositional base for the arguments against the existence of God, it undermines the evidences that are provided for the existence of God because to say there is no evidence for the existence of God suffices to say that the atheist's knows all possible proofs that God exists, which is logically absurd. This is what I meant with my ontological argument as a preface for the evidence of absence. There is enough rational proof for the existence of God and that's where Plantinga's argument comes into play. Philosophically, it is possible. Yes, you can flip the logic but I don't believe that's sufficient enough to prove the argument holistically false.  The difference here is I am saying that a maximally great being is necessarily possible and not simply possible. The reason for providing Plantinga's argument in the first place is because I am meaning to exhibit that there are no logical objections to the maximally great being unless the objector can prove that to be false. Rendering us back to premise 1. that it is possible. " - hes reply


Now this is where i am at the moment. now form my limited understanding it seems the major problem with this argument is that it uses two different meanings for the word possibility and only gets away with this argument by confusing them. the one type of possibility has to do with something that is either true or false but which the laws of logic cannot say one way or the other  . such as the possibility england will win tomorrows rugby match.
The other possibility seems to be a logical possibility, something that has always been either true or false and always will be either true or false and so then has to be neccesarily true or false. and the only reason we can call it a possibility is because we do not have the answer yet.
Now this is what i have come up with so far to answer back to what he replied just  and i would like to know if it is valid or not


 "  what i am  saying is that this  argument proves nothing. lets us  replace god with some other proposition which is possible and  if true is neccaserily true for all possible worlds  , but that we do not know the answer to. lets call it (x) and (x)=  the statement that the 12 billionth and first digit of pi is a 7. now we dont know the answer so it is possible that (x) is true  , and if true in any world it would be true in all worlds,so  if true it is neccaseraly true. now i will use your exact same argument
1. It is possible that (x) is true
2. If it is possible that(x) is true , then (x) is true  in some possible world.
3. If (x) is true in some possible world, then it true in every possible world.
4. If (x) is true  in every possible world, then it true in the actual world.
5. If (x) is true  in the actual world, then (x) is actualy true
6. Therefore, (x) must be true

Now we do not need to know the actual answer to be able to say this does not prove (x) is true. yes it is possible (x)  true, there is a 1/10 chance it is, but this does not prove it is true and we do not need to be able to prove  (x) is untrue to be able to say this does not prove anything."


Now this seems fine to me and the only objection i can think of is if , in modal logic, it is possible for worlds to exist where logical and mathematical truths are not true any longer. ie if there is a world where 2+2=563 or something

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Okay, yet I am still confused about one thing though.

The difference between these sentences: 1. It is possibly necessarily true that unicorns exist and 2. it is possible that unicorns exist. If I understand correctly by S5 in 1. you leave out the qualifier possibly and retain the sentence that it is necessarily true that unicorns exist and with 2. that (if it is possible that unicorns exist) it is necessarily possible that unicorns exist.

What does this mean with respect to the set of logically possible worlds? Well I would think that 1 then becomes unicorns exist in all logically possible worlds and 2 that is (necessarily) possible (but not necessary) that unicorns exist in all logically possible worlds.

What am I overlooking here?

In addition I must apologize to James, because it appears I stole the example from him. (In fact I am ashamed to admit that it is even worse than that because I didn't. I'm just busy with way too many things and I suck even at serial multitasking.)

So if there's nothing special about the "maximally great" being that allows this to work, then you can just shoe-horn in anything you want, like unicorns or winning the lottery, right? To make a generic example, it essentially becomes "if something is not demonstrably impossible, then it exists."

If so, I fail to see why experts consider this legitimate.  If anything, it just demonstrates a flaw in modal logic.  But maybe I'm still missing something.

So does the S5 axiom have legitimate uses in other contexts?  Is that why it is not rejected outright?

OK, I've been reading this page that Albert posted later in the thread.

Based on this:

Plantinga defines God as necessary... God, then, either does not exist or he exists in all possible worlds.

...

Axiom S5 requires a proposition that, if true, can only be necessarily true, and which is possibly true, to be necessary. In other words, Axiom S5 states that if something's definition only allows it to be true or to exist necessarily, then if there is a possible world in which it is true or exists, it is true or exists in all possible worlds.

It sounds like this argument only works for things that are by definition necessary (exist in all worlds), which would not apply to unicorns or winning the lottery, right?  Those things are just possible, but not "possibly necessary" like Plantinga's God.  If so, then you can't just substitute any old silly thing in this argument for "God" like a lot of people are trying to do.

"Few people without some knowledge of modal logic will know that arguments using modal logic are representative of a legitimate field of study."

Just reading this thread made my phreakin' head hurt! I am completely ignorant about a lot of subjects and modal logic is there at the top.

But I do have one simple honest question: Are there practical applications for modal logic? If so, what would be a layman's example? Thanks.

Also computer science to an extent I think. I'm pretty sure it's the Matrix.

[Thanks Greg for suggesting a few introductory books on modal logic. I am getting convinced that it'll be a good thing if I am to gain some basic understanding of this stuff.]

WTF, I gotta lay down after reading that...lol.

Simply reasoning something into existence without any credible evidence to support that claim (notice I didn’t say no evidence, because there’s plenty of poor “evidence” that gets thrown around) doesn’t make it believable. So, your friend has come to the conclusion, through the use of logic, that such a being does exist. I’d say fine, for the sake of argument, concede the point and ask him to present evidence that supports its existence. This argument alone isn’t proof of existence any more than your counter argument is proof of non-existence. Perhaps you should remind him that most atheists accept the (remote)possibility that some god may exist, but in the absence of convincing/credible evidence, have no reason to believe it.

The type of argument/ discussion you’re engaged in with your friend always seem like a pointless exercise in mental masturbation. Personally, I think the physical act is a more worthwhile pursuit.

Problem with "every possible world": one absolutely possible world would be one which is totally bereft of life of any sort, physical OR spiritual. I think that ***ks up the argument pretty badly right there.

Also, one perpetual problem with all ontological arguments is that whatever that being is is God. Now, is that Jehovah or Allah or Krishna or Ahura Mazda? You see, this is what renders the ontological argument unmoving for so many people, who don't want to be convinced that SOME deity exists, but that THEIR deity exists. That is one thing the ontological argument fails to do miserably.

You see, this is what renders the ontological argument unmoving for so many people, who don't want to be convinced that SOME deity exists, but that THEIR deity exists.

True, but it's an important step.  First prove there is *a* god, then prove that God is whichever one you prefer.  William Lane Craig (I know, bad example) does this: ontological argument to prove there is a god, then uses the "evidence" of Jesus' resurrection to "prove" it's the Christian God.

Yeah, but like I said, "one absolutely possible world would be one which is totally bereft of life of any sort, physical OR spiritual." The argument falls flat on its face right there.

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