Sobel spends much of the third chapter Logic and Theism evaluating the dialectical status of ontological arguments, and, in particular, whether ontological arguers are entitled to the premise that it is possible that there be a perfect being. I am simply going to take the occasion here to state my opinion on the matter.
There is a fundamental dialectical tension in the ontological arguments that start from this premise. If, on the one hand, necessary existence follows trivially from the stipulated definition of perfection, then the argument will beg the question as Sobel suggests that Anselm's argument does. That is, if necessary existence follows trivially, then no one who doesn't already believe that God necessarily exists will be willing to accept the premise. So in terms of having a good, non-question-begging argument for the (necessary) existence of God, it would be best to have a non-gerrymandered definition from which it is surprising that necessary existence follows. 'A perfect being' or 'a being than which none greater can be conceived' could fit the bill, depending on how 'perfection' and 'greatness' are defined.
The problem with this approach is that by showing that the definition unexpectedly leads to necessary existence, we've shown that not all of the definition's consequences are visible at first glance. Probably, the definition has other unexpected consequences, and one of those might be a contradiction. So if we fix the argument so that it doesn't beg the question, we create a different sort of problem with the justification of the premise.
As a result of these considerations, I don't think that ontological arguments can do very much to increase our credence in the existence of a perfect being. They might be able to do better at getting us from (mere) existence to necessary existence, and this would help to buttress cosmological arguments: if we could show that the non-existence of God was the same sort of contradiction as the non-existence of a number greater than 1 and less than 3, then theism would allow us to preserve a strong form of the Principle of Sufficient Reason, a principle which many people (theists and atheists alike) find intuitively appealing, and which is extremely difficult to preserve in a non-theistic system. I have discussed this line of thought before.
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