In my amateur philosophy club, my friend told me that modal ontological argument is false because its premise, It's possible that a perfect being exists, doesn't make sense. He argued that it is logically equivalent to say "it is possible that it is necessary", which means 'there exists at least one possible world in which all possible worlds have this objects in them.' So, according to his analysis, that premise make possible worlds in a possible world, which is absurd and makes a danger of infinite regress. But I think he misunderstood the argument. I think what actually that premise says is "there is at least one possible world that has a object which is in every possible world." I think this is implied when the argument says that "if something possibly necessarily exists, then it necessarily exists." Am I wrong?

Excellent question. It's great to hear that you belong to a philosophy club.

As I see it, if the modal-ontological argument fails, it's not because the locution "It is possible that it is necessary" is absurd or ill-formed or meaningless. The opening premise of the modal-ontological argument can be expressed without using the possible worlds idiom: There could have been a necessarily existing God (where "could have been" is construed as consistent with "is"). The idea is that even atheists are supposed to concede that a necessarily existing God is at least logically possible: logically speaking, there could have been such a thing (even if, according to atheists, there isn't).

Granted the possibility of a necessarily existing God, the argument then uses the modal principle "If it's possible that it's necessary that G, then G," letting "G" in this case stand for the proposition that God exists. Conclusion: God actually exists.

In my view, the argument can be challenged for assuming (1) the above-mentioned modal principle, which has many critics; and (2) the logical possibility of a necessarily existing God. Even philosophers who accept the principle and who regard the argument as sound, such as Alvin Plantinga, recognize that opponents can reasonably reject (2). Indeed, the monotheistic concept of a necessarily existing God is so logically powerful, so rich with potential implications, that it seems overhasty to assume that the concept harbors no internal contradictions.

Your friend seems to object to iterated modalities as such: "It could have been necessary," "It could have been possible," "It's necessarily possible," etc. I don't think they're defective locutions, but it's controversial what they imply. For details, you might consult this SEP entry.

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